The diseases of the heart are: 1, inflammatory affections; 2, organic diseases, or structural lesions; and 3, functional disorder. The inflammatory affections are distinguished from each other and named according to the particular structure inflamed. Inflammation of the serous membrane which covers the organ and lines the heart sac (pericardium) is called pericarditis. Inflammation of the membrane lining the cavities of the organ (endocardium) is called endocarditis. Inflammation of the substance of the organ (muscular and connective tissue) is called myocarditis or carditis. I. Inflammatory Affections. - Pericarditis. The inflammation in pericarditis may be either acute or subacute and chronic. Acute pericarditis is characterized by the same local morbid effects essentially as acute inflammation affecting other analogous serous membranes, as for example acute pleurisy. The inflammatory product called coagulable lymph or fibrinous exudation is found after death in more or less abundance, covering the inflamed membrane, together with the effused liquid or serum, the quantity of this varying in different cases, and holding in suspension flakes of lymph.

If recovery take place, the lymph and the serum disappear, and in place thereof new tissue is formed causing permanent adhesion of the heart to the pericardial sac, either wholly or in part. An entire obliteration of the space between the heart and this sac, by means of this newly formed tissue, is not incompatible with the continuance of life and health. Acute pericarditis may be produced by penetrating wounds or contusions of the chest. Exclusive of these so-called traumatic causes, the affection occurs in connection with acute articular rheumatism, or rheumatic fever. The affection is generally secondary to some other disease, most frequently rheumatism. It sometimes occurs in connection with either acute or chronic disease of the kidneys; also in some cases of pleurisy and pneumonia. It is a rare complication in cases of pyaemia, scurvy, erysipelas, and the eruptive and continued fevers. As a primary affection it is exceedingly rare. An attack of pericarditis is generally indicated by acute, lancinating pain, referred to the region Of the heart, increased by a deep inspiration. Its intensity varies considerably in different cases, being sometimes excruciating, and sometimes slight or even wanting.

There is also more or less tenderness on pressure over the region of the heart, and its action is notably increased. With these local symptoms are associated those pertaining to the system at large which accompany symptomatic fever. As the affection is almost always developed in connection with other diseases, the symptoms of the latter are of course combined with those of the former. After a time, which in some cases consists of only a few hours, the local symptoms are liable to be modified by the effects of the accumulation of liquid within the pericardial sac. The movements of the heart are restrained in proportion as the liquid is abundant and rapidly effused. The pulse may now become feeble and irregular, and the patient suffer from a distressing sense of oppression, which is increased by any muscular exertion or by emotional excitement. The compression of the heart may be the cause of death, which sometimes occurs suddenly after some exertion or excitement. - The diagnosis or recognition of pericarditis has been rendered prompt and positive by means of auscultation and percussion. Soon after the attack, the exudation of fibrine occasions a friction sound with the heart's movements, and this is proof of the existence of the disease.

Afterward, when considerable liquid has been effused into the sac, the friction sound may cease, but it is practicable to determine the presence and the quantity of liquid within the sac by physical signs, which are obtained by auscultation and percussion. The danger in cases of pericarditis depends, other things being equal, on the intensity of the inflammation, the quantity of exuded fibrine, and the amount of effused liquid. Aside from these conditions, much depends on the diseases with which it is associated. When developed in connection with rheumatism, it ends in recovery in the majority of cases; but occurring in connection with disease of the kidneys, with pleurisy or pneumonia, and in cases of pyaemia, it ends in death much oftener than in recovery. When death is not sudden, the disease destroys life by slow asthenia or exhaustion. - As regards treatment, acute pericarditis claims in general the measures indicated in other inflammatory affections. These measures, however, are in many cases to be modified by the circumstances pertaining to the diseases with which this is associated.

A very important fact proper to the affection, however, is that the source of danger is the weakness of the heart as a direct effect of the inflammation, and as caused by the pressure of liquid within the sac. This modifying fact contra-indicates active measures of treatment which in themselves tend to impair the power of the heart's action. Whenever the effusion of liquid is such as to compress the heart, measures having for their object' the absorption of the liquid are indicated. In the treatment of rheumatism an important object is to prevent the development of this complication; and clinical experience has shown that this object is promoted by the use of alkaline remedies. - Chronic pericarditis may be a sequel of the acute affection, or the inflammation may be subacute from the first. In some cases the inflammation continues with an abundant exudation of lymph, agglutinating the inner surfaces of the sac, and proving fatal by slow exhaustion. In other cases a large accumulation of liquid takes place, amounting to several pounds in weight; and to the exhaustion incident to the persistence of the inflammation is added the compression of the heart thus occasioned. In both varieties the disease, as a rule, proves fatal sooner or later.

A few cases have been reported in which the liquid has been removed by puncture of the chest, and relief of distressing symptoms has been thereby obtained. The removal of liquid from the pericardial sac through a very small canula by means of suction, or, as it is called, aspiration, can be effected without danger from the operation, and it remains to be ascertained whether in some instances recovery may not follow. - Endocarditis. In this affection the inflamed membrane is in contact with the blood contained within the cavities of the heart; hence, although fibrinous exudation takes place as in pericarditis, the exuded lymph is in a great measure washed away from the membrane and carried into the circulation. A portion, however, adheres to the membrane, roughening the surface in contact with the blood, and giving rise to an abnormal sound (an endocardial or bellows murmur), which is an important physical sign of the disease. Moreover, upon the little masses of lymph which adhere to the membrane coagulated fibrine from the blood contained in the cavities of the heart is apt to be deposited; and in this way are produced the so-called vegetations or warty growths which, being sometimes detached and carried into the arteries with the current of the blood, are arrested in vessels too small to allow of their further progress, become fixed, and occasion an obstruction which may lead to haemorrhage (haemorrhagic infarctions), and to the impairment of nutrition within a circumscribed area beyond the point at which the obstruction is seated.

These movable plugs or emboli, as they are termed, play an important part in affections proceeding from disturbance of the circulation and nutrition in different organs of the body, more especially the brain. (See Brain, Diseases of the.) The phenomena thus produced are treated of by medical writers under the head of embolism, and the study of these phenomena within the last quarter of a century has enlarged in no small degree the boundaries of pathological knowledge. The local effects of endocarditis are also of much importance as laying the foundation for progressive changes, especially in the valves of the heart, constituting what are called valvular lesions. The inflammation in endocarditis is generally limited to the left side of the heart, that is, to the endocardial membrane lining the left ventricle and the left auricle. - Like pericarditis, this is very rarely a primary disease; and in the great majority of cases it occurs in connection with acute articular rheumatism. It is evidently due to the same internal agency which in rheumatism causes the inflammation within the joints, this agent being a morbid principle in the blood, supposed to be lactic acid. Endocarditis occurs in a much larger proportion of cases of acute articular rheumatism than pericarditis.

These two diseases are associated whenever rheumatic pericarditis occurs; in other words, the latter affection rarely if ever occurs in rheumatism without the coexistence of endocarditis. The development of endocarditis is attended with little or no pain or other subjective symptoms referable to the heart; and hence the knowledge of its existence followed the application of auscultation to the study of diseases of the chest. The diagnosis rests wholly upon physical evidence obtained by auscultation. The roughening of the endocardial membrane within the left ventricle causes, as already stated, an adventitious sound or murmur, and the production of this murmur while a patient is under observation constitutes the proof of the presence of the affection. The diagnostic murmur accompanies the first sound of the heart, and is referable to the mitral valve. The immediate danger from endocarditis is slight; indeed, there is no immediate danger to life except from the formation within the heart of a clot (thrombosis) of sufficient size to arrest the circulation either within the heart or the large vessels. The liability to embolism has been already referred to, but this is seldom if ever directly fatal.

The affection, however, is by no means one of small importance, the danger being that valvular lesions may be the result. These lesions, progressively increasing, may at a period more or less remote, often after the lapse of many years, occasion death. - As regards treatment, endocarditis calls for no active measures. It is important that undue action of the heart be prevented as far as possible by enjoining mental and physical quietude, and relieved, if it exist, by soothing remedies. The alkaline treatment in cases of rheumatism is preventive to a certain extent of this affection, as it is of pericarditis. It is probable also that the judicious use of alkaline remedies moderates the intensity of the endocardial inflammation, and thereby diminishes its local effects. - Carditis, or Myocarditis. Inflammation of the substance of the heart, irrespective of the pericardium and the endocardium, is extremely rare. Suppurative inflammation, however, sometimes occurs, giving rise to an abscess in the walls of the organ or in the septum between the ventricles.

The pus contained in the abscess may be discharged into the pericardial sac, causing pericarditis, or into the ventricular cavity, causing purulent infection of the blood (pyaemia). Induration produced by a morbid increase of the tissue which unites together the muscular fibres, is another effect of inflammation seated in the walls of the organ. There are no means of determining during life the existence of carditis or myocarditis. II. Organic Diseases. The organic diseases or structural lesions to which the heart is liable relate, first, to the valves and orifices, and second, to the walls of the organ. - Valvular Lesions. In the great majority of cases these are seated in the left side of the heart, being either mitral or aortic, or in both the situations indicated by these terms. The changes embraced under the name valvular lesions are various, arising from morbid thickening or attenuation, calcification (formerly called ossification), rupture of valves, etc. The various changes, however, produce their evil results chiefly in two ways, namely, by affecting the valves so as to render them more or less incompetent to perform their functions, and diminishing the size of the mitral or aortic orifice so as to produce more or less obstruction to the passage of blood.

The lesions which render the valves incompetent permit the blood to flow backward or regurgitate, and hence they are sometimes distinguished as regurgitant lesions. On the other hand, those which diminish the size of the orifices prevent the free passage of the blood in its direct or onward course, and hence they are termed obstructive lesions. Not infrequently the lesions are such as to involve both regurgitation and obstruction at the same orifice. The lesions may be situated at either the mitral or the aortic orifice, or at both orifices; and in some instances one or both of the corresponding orifices in the right side of the heart, the tricuspid and the pulmonic, are the seat of analogous lesions. - Obstruction of the flow of blood through the orifices within the heart, and regurgitation, lead to enlargement of the organ and to various morbid effects in other organs. The effects of mitral lesions relate especially to the lungs. Owing to the congestion of the lungs induced by obstruction and regurgitation at the mitral orifice, the changes in the blood effected by respiration are impeded, whence the sense of want of breath which in certain cases of disease of the heart is the source of great distress.

Haemorrhage into the air tubes and into the air cells sometimes results from the congestion due to mitral obstruction. An effect of the persistent pulmonary congestion caused by mitral lesions is an over accumulation of blood in the cavities of the right side of the heart, and from this effect follows general dropsy. Aortic lesions interfere especially with the circulation throughout the arteries of the system; the immediate effect is to keep the left ventricle over-distended with blood. The regurgitant lesions in this situation render the supply of arterial blood to the heart itself insufficient, and more than any other involve the liability to sudden death. It is a popular impression that all organic affections of the heart involve this liability. This is far from being true; sudden death occurs in only a small proportion of cases. - Valvular lesions generally occur as a sequel of acute articular rheumatism. They often take place slowly, and for a long time they are latent as regards any symptoms of which the person affected is conscious. Their progress, as a rule, is unattended by pain, and in general it is not until they have induced a certain amount of enlargement of the heart that the evils just referred to begin to be apparent.

Not infrequently many years elapse before they give rise to any marked effects. When not a sequel of rheumatism, they may arise from changes in nutrition incident to old age, and they are sometimes due to syphilis. The valves of the heart may be imperfectly developed, or lesions may result from disease occurring in foetal life. These congenital lesions are oftener seated in the right than in the left side of the heart. In a considerable proportion of the cases of young children affected with organic disease of the heart, the primary lesions are congenital. - The diagnosis of valvular lesions has been rendered very complete by means of auscultation. With very rare exceptions, they give rise to adventitious sounds, or murmurs, the characters of which, as regards their situation, their transmission in different directions, and their relations to the heart sounds, enable the physician not only to determine the existence of lesions, but to localize them, and to distinguish between those which involve obstruction and regurgitation. Moreover, the normal heart sounds are modified in such a way as to afford information of the extent to which the valves are injure,d by the lesions.

Auscultation, indeed, enables the physician to determine the existence, the seat, and the character of valvular lesions, long before they have occasioned any apparent morbid effects of which the patient is conscious. - When valvular lesions have advanced sufficiently to produce obvious symptoms referable to either obstruction or regurgitation, or to both, they will destroy life sooner or later. The duration of life varies within wide limits; often a great amount of injury is tolerated for a long period. The lesions are irremediable, and therefore the treatment does not embrace recovery as an object; but much may be done by judicious management to relieve symptoms, to postpone serious evils, and to prolong life. The more important of the measures of management relate to a proper regulation of the habits of life as regards diet, exercise, etc. While excessive muscular exercise is to be avoided, such an amount as is taken without discomfort may be highly useful by improving the general condition of the system. While excesses in eating and drinking are hurtful, a deficient alimentation is not less so.

In brief, a great end of treatment is to render the system tolerant of the lesions as much and as long as possible, and this end is promoted by such a course of management, hygienic and medicinal, as conduces to the general welfare of the economy. - Enlargement of the Heart. This is of two kinds, namely, enlargement due to an increase of muscular structure, and enlargement from increased size of the cavities. The first is represented by greater weight of the heart, and is called hypertrophy; the second is distinguished as dilatation, and is represented by augmented volume, without necessarily any increase of weight. But whenever the heart is considerably enlarged, the two kinds of enlargement, as a rule, are combined; and when combined, an important distinction relates to the predominance of either the hypertrophy or the dilatation. Enlargement from an increase of muscular structure, without dilatation, is called simple hypertrophy; and enlargement solely from increased size of the cavities is called simple dilatation. Enlargement by hypertrophy is the result of an abnormal growth of the muscular structure; and the growth of this involuntary muscle takes place, just as voluntary muscles are made to grow, by long continued increased exercise.

An immediate effect of obstructive and regurgitant valvular lesions is an undue accumulation of blood in certain of the cavities of the heart; the organ is thereby stimulated to increased power of action, and in this consists the pathological connection between valvular lesions and hypertrophy. But hypertrophy has its limitations; like the voluntary muscles, the muscular structure of this organ can only grow to a certain extent. It is a noteworthy fact that hypertrophic growth of the heart under the circumstances noted, so far from being an evil, is a positive advantage. The muscular strength of the organ being augmented by its muscular growth, it is enabled better to carry on the circulation despite the difficulties pertaining to the valvular lesions. In this point of view, hypertrophy of the heart is conservative or compensating. When hypertrophy has reached its maximum, the undue accumulation of blood in the cavities of the heart leads to dilatation; the walls yield more and more to the distention. Thus, in general, hypertrophy precedes dilatation, the latter taking place after the muscular structure has increased to the extent of its ability to grow. Causes other than valvular lesions may give rise to enlargement by hypertrophy and dilatation.

Emphysema of the lungs, and sometimes other affections which impede the free circulation of blood through these organs, involve an over filling of the right ventricle with blood; and the mechanism of the hypertrophy and dilatation which follow is the same as when the enlargement is caused by valvular lesions. Enlargement also occurs in some cases of chronic disease of the kidneys, the probable explanation being that the circulation through the capillary vessels is impeded, and hence the heart is excited to increased muscular exertion. Enlargement of the heart in different cases has its primary seat in different portions of the organ, and the enlargement of certain portions predominates. Thus valvular lesions at the aortic orifice induce first and especially enlargement of the left ventricle; mitral lesions lead particularly to enlargement of the left auricle and of the right ventricle; pulmonary emphysema leads to enlargement of the right, and renal disease to enlargement of the left side of the heart. These facts are sufficiently explained by the immediate bearing of the causative conditions on the blood currents and the quantity of blood in the different cavities of the heart.

It is a question whether enlargement is ever a result of the disturbances of the heart's action embraced under the name functional disorder. Cases in which this causative relation exists are certainly extremely rare. - The symptoms and morbid effects of enlargement vary much according as hypertrophy or dilatation predominates, or as either exists without the other. The effects referable to hypertrophy are due to the increased power of the heart's action, proportionate to the increase of its muscular structure. If hypertrophy exist alone, or if it greatly predominate, this increased power is represented by an increased momentum of the blood in the arteries. Active congestion, more especially within the skull, is sometimes a consequence. This effect would be of much more frequent occurrence were it not that in most cases of hypertrophy there are valvular lesions which tend to diminish the quantity of blood sent into the arteries. Aortic and mitral lesions, either obstructive or regurgitant, have this tendency, and hence they are conservative as regards the prevention of active congestion of the brain and other organs. The effects of dilatation are the opposite of those referable to hypertrophy.

Dilatation involves weakness of the heart, and its ability to propel the blood through the arteries is lessened in proportion as the heart is dilated. This power of enlargement is not, like hypertrophy, compensatory or conservative, when associated with valvular lesions; on the contrary, many of the evils of organic disease of the heart are attributable to the weakness incident to it. Dilatation of the right side of the heart resulting from mitral lesions stands in immediate relation to general dropsy, and in a certain degree to the defective pulmonary circulation. Dilatation of the left ventricle resulting from aortic lesions renders this part more liable to become distended with blood, causing paralysis of the muscular walls and sudden death. - Enlargement of the heart and its extent are easily determined by means of physical signs furnished by the touch (palpation) and percussion. By the touch it is found that the apex beat is more or less lowered and carried to the left of its normal situation. Between the apex and the base of the organ are often found impulses not perceptible in health. By percussion the boundaries of the organ are readily ascertained in the great majority of cases.

The vocal resonance also, as heard with the stethoscope, enables the physician to define the limits to which the organ extends. Palpation and auscultation furnish signs by which predominant hypertrophy may be differentiated from dilatation. If hypertrophy predominate, the impulses of the heart as felt by the hand are strong, and often there is a heaving movement extending over the region of the heart. The first sound of the heart, over the apex, is abnormally loud, long, and booming. On the other hand, if dilatation be considerable or great, weakness of the organ is denoted by feeble impulses and by diminished intensity together with shortness and a valvular quantity of the first sound of the heart in the situation of the apex. With these physical signs are associated, on the one hand, symptoms and effects denoting a morbid increase of the power of the heart's action in hypertrophy, and on the other hand, in dilatation, those proceeding from a morbid feebleness of the organ. - Hypertrophy of the heart seldom calls for treatment with a view to lessen or remove this lesion. To do this would in general not be desirable were it practicable; and it is not practicable, because the hypertrophy is in most cases a result of conditions which of necessity continue.

It would not be desirable, inasmuch as the lesion protects against the evils which would otherwise flow from the causative conditions, and it is therefore a conservative provision against these evils. It is different with dilatation; it is desirable to prevent the progress of this kind of enlargement, and to obviate the weakness of the heart which it induces. Measures of treatment may do considerable toward the accomplishment of these objects. The heart may often be strengthened by dietetic, hygienic, and medicinal measures to promote assimilation and nutrition; and in as far as the muscular structure of the heart is rendered strong and vigorous, it is less likely to yield to the distention of the blood within its cavities and become more and more dilated. There are some remedies which seem to exert a direct tonic influence upon the muscular structure of the heart. Digitalis is preeminently a remedy of this character. By the judicious use of this remedy in conjunction with hygienic treatment, the heart sometimes regains in a measure the strength which has been impaired by dilatation, the distressing evils which have already ensued being greatly relieved. This latter statement has reference especially to dropsy and suffering from a sense of the want of breath.

Life may be often prolonged and the tolerance of the lesion promoted by appropriate management. - Atrophy of the Heart. This is a lesion the reverse of enlargement, the term denoting a morbid diminution of the size of the organ. The heart is greatly diminished in size, and in some cases of great emaciation its weight may be reduced to 4 1/2 oz. Under these circumstances the atrophy is in accordance with the general condition, and gives rise to no special morbid effects. Atrophy may be produced by the adhesions which result from pericarditis, and the diminished muscular power thus induced may give rise to morbid effects dependent on feebleness of the circulation. As the size of the heart can be determined with much precision by physical signs, the diagnosis of atrophy is practicable. From its infrequency, however, and the very small proportion of cases in which it has pathological significance, it is not a lesion of much importance. - Fatty Degeneration. This term is not applied to the deposit of fat upon the heart or between the muscular fibres of its walls. The organ is sometimes overloaded with fat in these situations, and is doubtless thereby burdened; but serious consequences never follow.

Fatty degeneration of the heart means a substitution of fat for the proper muscular substance, and this takes place without as well as with an increase of the adipose tissue of the organ, and is a lesion of serious import. It is evident that in proportion as fat is substituted for the muscular substance, the power of the heart's action must be weakened. If the structural change be considerable and extensive, morbid effects and danger arise from the enfeebled circulation. The pulse is small, compressible, irregular, and sometimes notably slow; there is breathlessness on exertion; the patient readily faints, and there is danger of death from over distention of the cavities of the heart. The lesion involves softening of the muscular structure, sometimes resulting in rupture. There are no special physical signs which denote this lesion, but auscultation and palpation show persistent weakness of the heart's action. The apex impulse is feeble or not appreciable, and the first sound of the heart is feeble, short, and valvular over the apex. The lesion may exist alone, or it may coexist with valvular lesions and enlargement; in the latter case its existence is not easily determined.

When it exists alone the diagnosis may be made with much positive-ness, taking the symptoms and signs which have been mentioned in conjunction with the following facts: Fatty degeneration occurs rarely before middle age; it exists more frequently, but by no means invariably, in connection with general obesity; and it is often accompanied by the fatty change in the cornea known as the arcus senilis. The treatment consists of a highly nutritious diet, into which fatty articles should enter sparingly, together with the employment of hygienic measures and remedies designed to give tone to and to invigorate the heart. The lesion is irremediable; that is, the fibres which have undergone degeneration are never restored to their normal condition. All that is to be hoped for from treatment relates to the tolerance of the affection for an indefinite period, and the relief of symptoms. - Miscellaneous Lesions. Softening of the muscular structure of the heart, irrespective of fatty degeneration, may occur in connection with the continued and eruptive fevers, scorbutus, pyaemia, and other diseases. It is due to disturbed nutrition, and is accompanied by great feebleness of the circulation.

Softening as thus produced is not irremediable; restoration takes place on recovery from the diseases to which it is secondary. - Rupture of the heart has been mentioned as an accident occurring in connection with fatty degeneration. It may occur also as a result of circumscribed suppurative inflammation in the muscular walls, and of aneurismal dilatation. It is one of the causes of sudden death. ' If the rupture be of sufficient size to admit of the free escape of blood into the pericardial sac, the loss of blood and the compression of the heart by the blood accumulating in the sac, prove immediately or quickly fatal. Sometimes, however, the opening is so small that death is slowly produced, and cases have been reported in which the orifice has been temporarily closed by a coagulum of blood, and the escape of blood in sufficient quantity to cause death has been delayed from one to two days. Cicatrization and recovery are perhaps not beyond the limits of possibility, but it may be doubted whether there is an authentic case on record. - Cancer, hydatid cysts, fibrous growths, calcareous deposits, and other affections involving serious lesions, are very rarely seated in the heart; and when they are, their existence cannot be determined during life. - Wounds of the heart with perforation of the walls are of necessity fatal, death taking place as in cases of rupture.

Foreign bodies, however, may remain imbedded in the muscular substance without giving rise to any serious inconvenience for an indefinite period. The writer has seen a specimen in which a pistol ball was found in the walls of the right ventricle, the patient having received the wound 20 years before his death, and the cause of death being an attack of pneumonia. III. Functional Disorder of the Heart. Under this name are embraced all kinds of disturbed action occurring irrespective of either inflammation or any structural lesion. The forms of functional disorder are various. A frequent form is that commonly known by the name palpitation, consisting of violent or tumultuous action, of which the patient is distressingly conscious, occurring in paroxysms very variable as regards their duration and their recurrence. In severe cases patients ofof the doctrine of the convertibility of forces, will be found in the article Correlation of Forces. The experiments of Rumford and Davy were made about 80 years ago, but were not at the time regarded as conclusive; nor were the more refined demonstrations of Thomas Young of the truth of Huygens's theory of light.

It seems to have required the later investigations of Fresnel, Cauchy, Malus, Mel-loni, Tyndall, Sir William Hamilton, and others, to adapt the undulatory theory to the explanation of all the phenomena of radiation, to render the mechanical demonstrations acceptable. It is interesting to observe the clearness with which Rumford and Davy so long ago stated their views upon the nature of heat. In a tract published in 1798, giving an account of his experiments at Munich, the former says: " It appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in the manner that heat was excited and communicated in these experiments, except motion." In a tract contained in a volume published at Bristol in 1799, Davy says: "Heat, then, or that power which prevents the actual contact of the corpuscles of bodies, and which is the cause of our peculiar sensations of heat and cold, may be defined a peculiar motion, probably a vibration of the corpuscles of bodies, tending to separate them." In his "Chemical Philosophy," published in 1812, he says: "The immediate cause of the phenomenon of heat, then, is motion, and the laws of its communication are precisely the same as the laws of the communication of motion." The dynamical theory of heat may therefore be stated in almost the words quoted above.

It holds that heat consists in the vibratory motion of the particles of matter, and that it may be produced by mechanical force, such as friction, percussion, or compression, or by the electric current; or that it may be communicated by the undulatory ether, the medium of radiation. Its communication from one body to another when they are in contact, or through a homogeneous body, from particle to particle, constitutes conduction. - Sources of Heat. Ac-cording to the nebular hypothesis of Laplace, heat is a primal force which caused all matter at one time to exist in a gaseous condition, which by the action of gravitation and other forces has been aggregated into masses assuming solid and liquid conditions. But the opinion has been advanced by J. E. Mayer and Waterson, and more recently elaborated by Helmholtz and Thomson, that the sun owes its heat to the force of gravitation acting upon the particles of matter, which at the beginning are assumed to have been at considerable distances from each other, and causing by their clashing together the evolution of heat. According to either theory, the sun is regarded as a vast storehouse of radiant heat from which the earth derives its supply, and has done for myriads of years, through most of the geologic ages.

Estimations have been made by Pou-illet which show that the sun emits a quantity of heat per hour equal to that which a layer of anthracite coal 10 ft. thick would yield in combustion. Chemical combination, including the combustion of fuel, is a secondary source of heat, originally derived from the sun, which furnished the energy necessary to the formation of the fuel. The intensity of heat produced by combustion attains its maximum in the oxyhydrogen blowpipe, in which a heat approaching 4000° F. is reached. Mechanical action, either in the form of compression, percussion, or friction, develops heat in quantities equivalent to the force converted into it. The electric current is another source of heat, and an example of the equivalent conversion of one force into another. When the current is produced by the rotation of magnets, there is a conversion of the mechanical force expended in effecting the rotation into electricity, and this under proper circumstances into heat. When the current is produced by the chemical action of a battery, the origin of whatever heat is obtained may be considered as arising from the combustion of the elements in the battery.

The intensity of heat obtained by the electric current is considerably higher than that of the oxyhydrogen blowpipe, but the amount cannot be stated with any degree of accuracy. - General Effects of Heat. The most obvious effects of heat on matter are to cause it to expand and to assume different states, as the solid, liquid, and gaseous. Thus, under the ordinary pressure of the atmosphere, water at a temperature below 32° F. is a solid; between 32° and 212° it is a liquid; and above 212° it is in a gaseous condition. With a few exceptions, an increase of heat in bodies causes them to expand. Thus, a metallic bar which has a diameter just sufficient to enable it to be passed through an orifice, will by being heated become too large; the heat vibrations have been intensified, and the bar has increased in bulk. In what this slight expansion consists it is impossible to say with certainty; whether the molecules require greater space in which to vibrate, or whether they expand by a slight separation of the atoms of which they are composed, is not known; it is possible that both causes unite to produce the effect.

When, however, a body by the application of heat is converted into vapor, a repulsive force is generated between the particles when a certain temperature is reached which produces a far different phenomenon from ordinary expansion; an active repulsion has been generated, which may exist between molecules, as in the vapor of water or alcohol, or between the atoms. Moreover, the atoms composing molecules may be completely separated, molecular disintegration, and consequently decomposition of the substance, taking place. Heat when it has such an effect is called the heat of dissociation, and is required in different degrees by different bodies. (See Dissociation.) Thus, the atoms composing the molecules of oxide of silver are dissociated at a comparatively low temperature, while the molecules of water require a white heat to produce dissociation between the hydrogen and oxygen atoms composing it. The elements of oxide of lead are also dissociated by a comparatively low temperature when oxygen is excluded, while to cause dissociation between the atoms of oxygen and iron in oxide of iron requires so high a heat that separation is difficult, except in the presence of a third body, an example of which is the reduction of iron ore by charcoal or anthracite, in which, however, the heat of dissociation is not reached.

Advantage is taken in the arts of the expansion which heat produces in bodies for various industrial operations, such as the placing of tires on wagon wheels and the moving of immense weights or resistances, as the drawing toward each other of the walls of buildings. (See Expansion.) The construction of instruments for the measurement of heat is also founded upon the property of expansion. (See Pyrometer, and Thermometer.) It sometimes occurs that at the temperature at which a liquid solidifies there is expansion instead of contraction, as in the solidification of iron and bismuth, and also of water, a familiar example, in which the expansion is made obvious in the floating of the less dense ice; and mechanical advantage is often taken of this property of expansion during congelation to rend asunder masses of rocks or iron vessels. The phenomena and philosophy of combustion are treated under the title Combustion; the expanding force of heat, particularly with reference to liquids and solids, under Boiling Point and Expansion; the transmission of radiant heat, especially in connection with the diathermanous properties of different bodies, under Diathermancy; the generation of heat by mechanical means and by electricity, and its correlation with these forms of energy, under Correlation of Forces, Electricity, Friction, and Galvanism; and the causes of solar heat and its continuance or dissipation, under Sun. The remainder of this article will therefore be principally devoted to a consideration of the more general laws of radiant heat, of the conduction of heat, and of specific and latent heat.

I. Radiant Heat. The undulatory theory of radiation will be treated in the article Light, and only such of its laws will be given here as are necessary for the treatment of the subject, and some of the reasons which indicate the identity of the two forces. A beam of light from the sun, or from any highly incandescent body, consists of a great number of rays propagated by transverse vibrations in the ethereal particles. These vibrations are of variable amplitude, corresponding to the particular kinds of rays, and these rays have the property of being refracted when passing from one medium to another in an oblique direction, as when passing from air into glass, and again from glass into air or any other medium. Those rays which consist of vibrations of greater amplitude have been found the least refrangible, and also to be those which in a greater degree than the others produce the effects of heat. When a beam of light is dispersed by a triangular prism made of rock salt, a highly diathermanous substance, there is formed a luminous spectrum of various colors in which heat is more or less distributed, abounding more in the red or least refrangible light than in other portions; but far more in that part of the spectrum which is composed of invisible rays of still less refrangibility than the red.

It is estimated that the amount of heat contained in the invisible or non-luminous part beyond the red rays is more than seven times as great as that in the luminous part. Here, then, is a proof that rays of light and rays of heat are transmitted together in ordinary radiation of compound light. Now, if they are found to travel with the same velocity, their identity becomes probable, and this is shown by the fact that during an eclipse of the sun, at the conclusion of total obscuration, heat makes its appearance simultaneously with the rays of light; and finally, when it is found that the rays of light and heat observe the same laws of reflection, refraction, interference, and polarization, the conclusion is irresistible that the only difference between the two is that the less refrangible rays possess the greater heating power. Radiation of both light and heat is propagated in straight lines in a homogeneous medium, and unlike sound may be transmitted through a vacuum, a fact which indicates that it employs a different medium.

If a sphere of glass, a, fig. 1, have a thermometer, b, sealed into it, with its bulb placed in the centre of the sphere, and if the air be exhausted through the tube c, which is afterward closed by the flame of a blowpipe, and then the sphere be surrounded by a heated body, as a piece of tin foil, the thermometer will indicate a rise of temperature. The radiation of heat follows three important laws: 1. Its intensity is proportional to the intensity of the source. 2. It is inversely as the square of the distance. 3. Its intensity is less in proportion to the obliquity of the surface of the body emitting the rays. The first law is demonstrated by placing of the doctrine of the convertibility of forces, will be found in the article Correlation of Forces. The experiments of Rumford and Davy were made about 80 years ago, but were not at the time regarded as conclusive; nor were the more refined demonstrations of Thomas Young of the truth of Huygens's theory of light. It seems to have required the later investigations of Fresnel, Cauchy, Malus, Mel-loni, Tyndall, Sir William Hamilton, and others, to adapt the undulatory theory to the explanation of all the phenomena of radiation, to render the mechanical demonstrations acceptable.

It is interesting to observe the clearness with which Rumford and Davy so long ago stated their views upon the nature of heat. In a tract published in 1798, giving an account of his experiments at Munich, the former says: " It appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in the manner that heat was excited and communicated in these experiments, except motion." In a tract contained in a volume published at Bristol in 1799, Davy says: "Heat, then, or that power which prevents the actual contact of the corpuscles of bodies, and which is the cause of our peculiar sensations of heat and cold, may be defined a peculiar motion, probably a vibration of the corpuscles of bodies, tending to separate them." In his "Chemical Philosophy," published in 1812, he says: "The immediate cause of the phenomenon of heat, then, is motion, and the laws of its communication are precisely the same as the laws of the communication of motion." The dynamical theory of heat may therefore be stated in almost the words quoted above.

It holds that heat consists in the vibratory motion of the particles of matter, and that it may be produced by mechanical force, such as friction, percussion, or compression, or by the electric current; or that it may be communicated by the undulatory ether, the medium of radiation. Its communication from one body to another when they are in contact, or through a homogeneous body, from particle to particle, constitutes conduction. - Sources of Heat. Ac-cording to the nebular hypothesis of Laplace, heat is a primal force which caused all matter at one time to exist in a gaseous condition, which by the action of gravitation and other forces has been aggregated into masses assuming solid and liquid conditions. But the opinion has been advanced by J. R. Mayer and Waterson, and more recently elaborated by Helmholtz and Thomson, that the sun owes its heat to the force of gravitation acting upon the particles of matter, which at the beginning are assumed to have been at considerable distances from each other, and causing by their clashing together the evolution of heat. According to either theory, the sun is regarded as a vast storehouse of radiant heat from which the earth derives its supply, and has done for myriads of years, through most of the geologic ages.

Estimations have been made by Pou-illet which show that the sun emits a quantity of heat per hour equal to that which a layer of anthracite coal 10 ft. thick would yield in combustion. Chemical combination, including the combustion of fuel, is a secondary source of heat, originally derived from the sun, which furnished the energy necessary to the formation of the fuel. The intensity of heat produced by combustion attains its maximum in the oxyhydrogen blowpipe, in which a heat approaching 4000° F. is reached. Mechanical action, either in the form of compression, percussion, or friction, develops heat in quantities equivalent to the force converted into it. The electric current is another source of heat, and an example of the equivalent conversion of one force into another. When the current is produced by the rotation of magnets, there is a conversion of the mechanical force expended in effecting the rotation into electricity, and this under proper circumstances into heat. When the current is produced by the chemical action of a battery, the origin of whatever heat is obtained may be considered as arising from the combustion of the elements in the battery.

The intensity of heat obtained by the electric current is considerably higher than that of the oxyhydrogen blowpipe, hut the amount cannot be stated with any degree of accuracy. - General Effects of Heat. The most obvious effects of heat on matter are to cause it to expand and to assume different states, as the solid, liquid, and gaseous. Thus, under the ordinary pressure of the atmosphere, water at a temperature below 32° F. is a solid; between 32° and 212° it is a liquid; and above 212° it is in a gaseous condition. With a few exceptions, an increase of heat in bodies causes them to expand. Thus, a metallic bar which has a diameter just sufficient to enable it to be passed through an orifice, will by being heated become too large; the heat vibrations have been intensified, and the bar has increased in bulk. In what this slight expansion consists it is impossible to say with certainty; whether the molecules require greater space in which to vibrate, or whether they expand by a slight separation of the atoms of which they are composed, is not known; it is possible that both causes unite to produce the effect.

When, however, a body by the application of heat is converted into vapor, a repulsive force is generated between the particles when a certain temperature is reached which produces a far different phenomenon from ordinary expansion; an active repulsion has been generated, which may exist between molecules, as in the vapor of water or alcohol, or between the atoms. Moreover, the atoms composing molecules may be completely separated, molecular disintegration, and consequently decomposition of the substance, taking place. Heat when it has such an effect is called the heat of dissociation, and is required in different degrees by different bodies. (See Dissociation.) Thus, the atoms composing the molecules of oxide of silver are dissociated at a comparatively low temperature, while the molecules of water require a white heat to produce dissociation between the hydrogen and oxygen atoms composing it. The elements of oxide of lead are also dissociated by a comparatively low temperature when oxygen is excluded, while to cause dissociation between the atoms of oxygen and iron in oxide of iron requires so high a heat that separation is difficult, except in the presence of a third body, an example of which is the reduction of iron ore by charcoal or anthracite, in which, however, the heat of dissociation is not reached.

Advantage is taken in the arts of the expansion which heat produces in bodies for various industrial operations, such as the placing of tires on wagon wheels and the moving of immense weights or resistances, as the drawing toward each other of the walls of buildings. (See Expansion.) The construction of instruments for the measurement of heat is also founded upon the property of expansion. (See Pyrometer, and Thermometer.) It sometimes occurs that at the temperature at which a liquid solidifies there is expansion instead of contraction, as in the solidification of iron and bismuth, and also of water, a familiar example, in which the expansion is made obvious in the floating of the less dense ice; and mechanical advantage is often taken of this property of expansion during congelation to rend asunder masses of rocks or iron vessels. The phenomena and philosophy of combustion are treated under the title Combustion; the expanding force of heat, particularly with reference to liquids and solids, under Boiling Point and Expansion; the transmission of radiant heat, especially in connection with the diathermanous properties of different bodies, under Diathermancy; the generation of heat by mechanical means and by electricity, and its correlation with these forms of energy, under Correlation of Forces, Electricity, Friction, and Galvanism; and the causes of solar heat and its continuance or dissipation, under Sun. The remainder of this article will therefore be principally devoted to a consideration of the more general laws of radiant heat, of the conduction of heat, and of specific and latent heat.

I. Radiant Heat. The undulatory theory of radiation will be treated in the article Light, and only such of its laws will be given here as are necessary for the treatment of the subject, and some of the reasons which indicate the identity of the two forces. A beam of light from the sun, or from any highly incandescent body, consists of a great number of rays propagated by transverse vibrations in the ethereal particles. These vibrations are of variable amplitude, corresponding to the particular kinds of rays, and these rays have the property of being refracted when passing from one medium to another in an oblique direction, as when passing from air into glass, and again from glass into air or any other medium. Those rays which consist of vibrations of greater amplitude have been found the least refrangible, and also to be those which in a greater degree than the others produce the effects of heat. When a beam of light is dispersed by a triangular prism made of rock salt, a highly diathermanous substance, there is formed a luminous spectrum of various colors in which heat is more or less distributed, abounding more in the red or least refrangible light than in other portions; but far more in that part of the spectrum which is composed of invisible rays of still less refrangibility than the red.

It is estimated that the amount of heat contained in the invisible or non-luminous part beyond the red rays is more than seven times as great as that in the luminous part. Here, then, is a proof that rays of light and rays of heat are transmitted together in ordinary radiation of compound light. Now, if they are found to travel with the same velocity, their identity becomes probable, and this is shown by the fact that during an eclipse of the sun, at the conclusion of total obscuration, heat makes its appearance simultaneously with the rays of light; and finally, when it is found that the rays of light and heat observe the same laws of reflection, refraction, interference, and polarization, the conclusion is irresistible that the only difference between the two is that the less refrangible rays possess the greater heating power. Radiation of both light and heat is propagated in straight lines in a homogeneous medium, and unlike sound may be transmitted through a vacuum, a fact which indicates that it employs a different medium.

If a sphere of glass, a, fig. 1, have a thermometer, b, sealed into it, with its bulb placed in the centre of the sphere, and if the air be exhausted through the tube c, which is afterward closed by the flame of a blowpipe, and then the sphere be surrounded by a heated body, as a piece of tin foil, the thermometer will indicate a rise of temperature. The radiation of heat follows three important laws: 1. Its intensity is proportional to the intensity of the source. 2. It is inversely as the square of the distance. 3. Its intensity is less in proportion to the obliquity of the surface of the body emitting the rays. The first law is demonstrated by placing a metallic cubical vessel at a certain distance from the blackened bulb of a thermometer,.and filling it successively with water at different temperatures, as for instance at 20°, 30°, and 40°; the temperatures indicated by the thermometer will be in the same ratio as those of the vessel containing the water. The second law follows from the geometrical principle that the surface of a sphere increases as the square of its radius.

Let c, fig. 2, be a centre of radiation; it will emit a certain number of rays, all of which will fall upon the inner surface of the sphere a b, or in the absence of this, upon the inner surface of the sphere d e, which has a radius twice as great as a b. Therefore the same amount of heat will fall upon either of the spheres. But the outer sphere has a surface four times as great as the inner one; therefore it receives upon the same extent of surface only one fourth as much heat. The same law may be demonstrated experimentally by a method invented by Tyndall. He placed a thermo-electric pile, S, fig. 3, in front of a rectangular vessel filled with hot water and having its face coated with lampblack. The pile is placed in the small end of a hollow cone, having its inner surface blackened, to prevent reflection. The distance of the pile from the vessel may be changed, but the quantity of heat received will be the same. If the distance at S' is twice that at S, the surface of the circle A' B', whose rays fall upon the pile at S', will have twice the radius and four times the surface of the circle A B, whose rays fall upon the pile at S. The third law is demonstrated as follows : Place a cube, a, fig. 4, filled with hot water, in front of a thermo-electric pile, P, and also place a screen, S S, with an opening, between the cube and the pile.

If the cube is first placed with its face perpendicular to the rays r,r, and is then turned upon its axis without changing the distance of the centre of its face, but giving it an oblique position, the amount of heat indicated by the pile will remain the same, although rays from a greater extent of surface on the cube will pass through the opening in the screen. All bodies are regarded as possessing a certain degree of that molecular motion which constitutes heat, and as always emitting rays of heat, no matter what their temperature may be. Every body is constantly receiving rays of heat from all other bodies within the limits of radiation, and is at the same time returning rays of heat to these bodies. But the hotter bodies emit rays of greater intensity than those which they receive, so that they all have a tendency to arrive at a condition of equilibrium. This is called the doctrine of exchanges, and was proposed by Prevost, a professor at Geneva about the year 1790, under the name of the " theory of mobile equilibrium of temperature." If a body could be so placed that it should continue to radiate more heat than it absorbed, there would come a time when its vibrations would cease, and it would possess no heat whatever; in other words, it would arrive at a state of absolute zero.

Modern physicists have assumed such a theoretical zero, and have calculated it to be at 459.13° below the zero of Fahrenheit's scale, or 272.85° below that of the centigrade. Newton was the first to enunciate a law of cooling, which was that "the quantity of heat lost or gained by a body at each instant is proportional to the difference between its temperature and that of the surrounding medium;" but it has been found not to be general, and only applies when the differences of temperature are not more than 15° or 20° C.; beyond that the loss or gain is greater than the law requires. No definite results were obtained till Dulong and Petit made a series of elaborate investigations, in which they placed the thermometer both in vacuo and in air. A large thermometer was used, containing about three pounds of mercury, and was placed in the centre of a hollow globe of thin copper having its interior surface covered with lampblack, and kept at a uniform temperature by immersion in a vessel of water, the bulb of the thermometer being hotter than the globe.

The following are the results obtained when the globe was at the temperature of melting ice:

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Fig. 1.

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Fig. 2.

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Fig. 1.

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Fig. 2.

Law of Inverse Squares.

Fig. 3. - Law of Inverse Squares.

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Fig. 4.

Velocity Of Cooling At Different Temperatures

Excess of temperature, in degrees F.

Velocity of cooling per minute.

432°...............................

.... 10.69°

396...............................

.... 8.81

860...............................

.... 740

324...............................

.... 6.10

288...............................

.... 4.89

252 ...............................

.... 3.83

216...............................

.... 3.02

180...............................

.... 2.30

144 ...............................

.... 1.74

It is thus shown that the velocity of cooling at 360° is more than three times as much as at 180°. It was found by Dulong and Petit that the velocity of cooling in a vacuum for a constant excess of temperature increases in a geometrical progression when the temperature of the surrounding air increases in an arithmetical progression, and that the ratio of this progression is the same whatever may be the excess of temperature. The experiments of MM. Provostaye and Desains confirm the results of Dulong and Petit. Radiation being the propagation in the luminiferous ether of undulations in consequence of molecular vibrations in the radiating body, it would be expected that different bodies would have different powers of radiation, and experiment shows this to be true. The apparatus used by Sir John Leslie is represented in fig. 6, and is the same that he employed for determining the reflecting powers of bodies. In experimenting on radiation, the plate d may be removed. The cube a has its sides coated with different substances, which may be turned at pleasure toward the mirror b, and the bulb of a differential thermometer may be placed in the focus I. Calling the radiating power of lampblack 100, he found that of other substances as follows:

Lampblack............. 100

Whiting................ 100

Paper.................. 98

Sealing wax............. 95

White glass............. 90

Varnished lead........... 45

Mercury................. 20

Polished lead............. 19

Polished iron............ 15

Tin, gold, silver.......... 12

It is commonly supposed that color has much influence on the radiating and absorbing power of bodies, but this is only true of luminous heat. If the cube used in the above experiment is filled with hot water, and three of its sides are covered, one with white, another with red, and another with black velvet, all of the same texture, the fourth of polished copper being left uncovered, it will be found that the three velvet sides will radiate alike, the naked side radiating the least. This shows that texture or molecular structure, rather than color, confers radiating power upon surfaces, for obscure heat. The power of a body to absorb heat is precisely proportional to its power of radiation; or in other words, its power of propagating undulations in the ether is equal to its power of accepting motion from the undulations of the ether, and is generally possessed in a greater degree by opaque than by transparent bodies, although there are remarkable exceptions, as will be seen by reference to the article Diathermancy. The method employed by Leslie in determining the absorbing powers of bodies was to cover the bulb of the differential thermometer, fig. 6, with the substance to be experimented upon, and place it in the focus, removing the plate d.

Tyndall has made elaborate researches upon the radiating and absorbing powers of gases, vapors, and flames, and has found them proportional when the same sources of heat were employed, and inversely proportional to their transmitting powers; but he also finds these properties to vary with the sources of heat. - Reflection of Heat. That dark heat rays are capable of reflection, and that they obey the same laws as the luminous rays may be shown by placing a metallic ball, A, fig. 5, heated below redness, i in the focus of a concave mirror, B C, and the bulb of a thermometer, D, in the focus of a mirror, E F, opposite and at a distance. The temperature indicated by the thermometer will approach that of the ball, but if either thermometer or ball is removed from its position, the temperature will fall. The following method was employed by Sir John Leslie to determine the heat-reflecting powers of different substances. The source of heat, which may be a cubical vessel filled with hot water, or a metallic ball, a, fig. 6, is supported by a sliding standard at a distance in front of a concave metallic reflector at b. The focus of the mirror is at c for the divergent rays which come from the source of heat, but a reflecting plate d reflects them to l, where the focus is really formed.

It is obvious that the heat at the focus l will be in proportion to the reflecting power of the plate d. By using plates of different materials he ascertained their reflecting power. Calling polished brass 100, he obtained the following results:

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Fig. 5.

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Fig. 6.

Brass.................. 100

Silver.................. 90

Tin..................... 80

Steel................... 70

Lead.................... 60

Amalgamated tin........ 10

Glass.................... 10

Lampblack.............. 0

This shows that the metals which are the best reflectors of light are also the best reflectors of heat. Moreover, when it is remembered that white light, which contains all the rays of the solar spectrum (those of dark heat, those of luminous heat, and those of actinism), is reflected from polished surfaces without any decomposition, we have another proof of the fact that the laws of reflection are the same for each. The reflecting power also of a surface for heat and for light is found by experiment to be the same, allowance being made for errors; and the reflecting power of different substances varies with the angle of incidence in the same degree for heat as for light. In glass it increases rapidly with the angle of incidence, while in metals it increases slowly. It is also found that heat is diffused and scattered by the same surfaces in the same proportion that light is. II. Conduction of Heat. If a copper rod, fig. 7, is placed on supports and a flame applied at one end, heat will flow along it toward the other end, and the rate may be measured by thermometers having their bulbs placed in cup-shaped holes containing mercury along the upper side of the bar.

This was the method of Despretz, who made the first important series of experiments on the subject.

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Fig. 7.

If an iron bar is substituted for the copper, the rate of flow will be much less, and a bar of platinum will be found a poorer conductor than one of iron. But the results of Despretz have been found by the later" experiments of Wiedemann and Franz not to be perfectly accurate. The results obtained by these investigators are given in the following table, which also gives the electric conductions of the same metals, according to Riess and Lenz, these being very nearly the same as for heat, a fact which was first shown by Forbes :

Rates Of Thermal And Electric Conductivity

METALS.

THERMAL CONDUCTIVITY.

ELECTRIC CONDUCTIVITY.

Wiedemann and Franz.

Riess.

Lenz.

Silver.............

100.0

100.0

100.0

Copper...........

73.6

66.7

73 3

Gold.............

53.2

59.0

58.5

Brass.......

23.6

18.4

21.5

Tin...............

14.5

10.0

22.6

Iron.............

11.9

12.0

13.0

Lead.............

8.5

7.0

10.7

Platinum.........

6.4

105

10.3

Bismuth.........

1.8

1.9

One of the sources of error in Despretz's experiments was the employment of the holes in the bar containing mercury, and another, a want of sensitiveness in the thermometer. Wiedemann and Franz used smooth rods, and measured the temperature with a thermo-electric pile and galvanometer. Marble and mineral substances generally are poorer conductors than any of the metals, and porcelain and glass are still poorer. The property of the thermal conductivity of metals is the basis of the invention of Davy's safety lamp for miners. (See Lamp.) The unequal conductivity of metals and other bodies is the cause of an interesting phenomenon, which is beautifully exhibited by what is known as Trevelyan's experiment, but which had been previously observed when a hot metal of good conducting power was laid against a cold one, a comparatively poor conductor, particularly if it had considerable expansion, as a copper brazing iron laid upon a block of cold lead. When the heated metal can readily acquire a slight rocking motion, the experiment succeeds the best. Tre-velyan's apparatus consists of a "rocker" made of brass, having a longitudinal groove, and lying upon the cylindrical surface of a block of lead.

When the rocker is heated and placed upon the lead, the ridges on each side of the groove are alternately thrown upward by the expansion in those parts of the surface of the lead which are heated by coming in contact with the hot brass, and thus a series of vibrations having a musical tone is produced. The reason why the heated metal should be a good conductor is that its surface may be kept hot uniformly with the mass, and thus be in a condition to impart sufficient heat to the surface of the lead at every moment. The advantage of employing lead as the other metal consists in its being capable of considerable expansion by heat, and in its being a poor conductor, so that in a moment the superficial portions may acquire enough heat to cause the requisite expansion to throw the rocker into vibrations. The same effect may be produced if, instead of a block of lead, one of stone is covered with a thin sheet of metal which is a good conductor, the condition required being one favorable to the rapid expansion of the surface, as was shown by Faraday. Other materials be-sides metals may be used, as various rocks and minerals. - Liquids are almost non-conductors of heat, as may be shown by pouring a small quantity of alcohol upon the surface of water in a tumbler and igniting it; along time will elapse before the upper layers of the water become sensibly heated.

Despretz employed an apparatus which consisted of a cylindrical wooden vessel about 3 ft. in height and 8 or 10 in. in diameter, which was filled with water. Through the sides of the cylinder 12 thermometers were placed, with their bulbs one above another in the axis of the column of water. A metal box, which was kept filled with water at 212° F., rested upon the top of the column of water. In this manner he found that the conductivity of heat for liquids follows the same law as for solids, but is much more feeble, the conductivity of water being only about 1/95 that of copper. Liquids are readily heated by convection. When heat is applied beneath vessels containing them, the stratum next the bottom expands by heat, and in rising the particles communicate their excess of heat to those through ■which they pass. Gases become heated in the same manner; they are exceedingly bad conductors, but from the mobility of their particles it is difficult to arrive at satisfactory results as to their conducting power. Porous substances containing confined air are bad conductors of heat, wherefore the walls of well built dwellings intended to exclude the heat of summer and the cold of winter are divided into partitions containing confined air.

Plaster of Paris, on account of its porosity after setting with water, and its non-combustibility, is used for filling between the plates of fire-proof safes; and the efficiency of porous garments in protecting the body against cold and heat is a matter of common observation. There is a remarkable exception to the non-conductivity of gases in the case of hydrogen, which, although the lightest of all of them, is by comparison far the best conductor of heat. This is proved by the following experiment: If a fine platinum wire is passed through a glass tube, as shown in fig. 8, and its two ends placed in connection with the poles of a galvanic battery, it will become incandescent on the passage of a moderate galvanic current, if air or any gas besides hydrogen is passed through the tube, though not to the same degree as in a vacuum; but if hydrogen gas is passed through the tube, the incandescence disappears in consequence of the heat being conducted away. III. Specific Heat. The first important experiments upon the specific heat of bodies were made by Dr. Black in the latter part of the 18th century, and the idea of measuring specific heat originated with him.

If two equal measures of water are placed in separate vessels of the same material, all being at the same temperature, and there is immersed in one an iron ball of a certain weight, and of a temperature higher than that of the water, and in the other a quantity of mercury of equal weight and temperature, after a time each of the vessels with their contents will have come to an equilibrium; but it will be found that the contents of the vessel in which the iron was placed have a higher temperature than the other, showing that the iron has communicated to the water a greater quantity of heat than the mercury. If the iron ball and the mercury had been colder than the water, on the attainment of equilibrium the water containing the iron would have been colder than that which contained the mercury. The amount which a body is thus capable of imparting or absorbing while rising or falling through a certain range of temperature is called its specific heat. The term first used to denote this property was "capacity for heat," and was introduced by Irvine, a pupil of Dr. Black. The term specific heat, according to Whewell, was proposed by Wilcke, a Swedish chemist, and according to others by Gadolin, of Abo, in 1784. If, in the experiment just mentioned, instead of an iron ball, an equal weight of water at the same temperature had been used, the quantity of heat imparted to the water already in the vessel would have been very much greater.

If equal weights of water at different temperatures are mingled, the resulting temperature will be a mean between the two; but when equal weights of iron and water at different temperatures are placed together, the resulting temperature will be nearer that of the water. In making experiments in specific heat, it is necessary to adopt some unit of measure, of which several are employed. The gramme degree (centigrade) is the quantity of heat required to raise one gramme of water 1° C.; the kilogramme degree, sometimes called a calorie, is the heat required to raise one kilogramme of water 1° C.; and the pound degree is the amount required to raise one pound avoirdupois of water 1° F. or 0. - Three methods have been employed for determining specific heat: 1, the method of fusion of ice; 2, the method of mixtures; and 3, the method of cooling. 1. The method of fusion of ice. This was employed by Black, and simply consisted in making a dee]) cavity in a block of ice, fig. 9, placing the substance to be experimented on in it, and closing the cavity with a cover of ice.

The substance is raised to a certain temperature, then introduced, and when cooled to zero is removed, and both it and the cavity wiped with a cloth of known weight; the increase in weight shows how much of the ice has been melted. Now, as will be seen further on, it requires as much heat to convert a pound of ice at 32° to a pound of water at 32°, as it does to raise a pound of water from 32° to 174°; therefore water at 32° contains 142° more heat than ice at the same temperature. Let m denote the weight of water derived from the ice in the above experiment, w the weight of the body under experiment, s its specific heat, and t the number of degrees it has fallen; then there will result the following equation: w t s =142m; or s = 142m/wt; from which formula the specific heat of any substance is readily ascertained. A modification of this apparatus, which gave more accurate results, was devised by Lavoisier and Laplace, and called an ice calorimeter, of which fig. 10 shows a perspective and a sectional view.

It consists of three concentric tin vessels, the inner one for holding the body under experiment, while the two others contain pounded ice, that in the outer one to prevent external influence, that in the middle one to measure the heat given by the body under experiment. Stopcocks are supplied to each, that connected with the middle vessel being for the purpose of drawing off the water which has been produced by the action of the experimental body. The manner of conducting the experiment is similar to that employed with the block of ice. The principal source of error is the difficulty of estimating the quantity of ice which has melted, as more or less water will adhere to the lumps. Bunsen has devised a calorimeter especially adapted to cases in which only small quantities are experimented upon. A test tube, a, fig. 11, which receives the substance to be tested, is fixed in the larger leg of a wide U-shaped tube, b, c, the part ft, containing the test tube, being filled with water, and the rest with mercury. A graduated smaller tube, d, open at the top, is adjusted to the top of the leg c, for the purpose of noting the rise or fall of the mercury in this leg, which it is obvious will be effected by the expansion or contraction of the contents of the leg ft.

In making the experiment, a is surrounded by a freezing mixture and the water frozen. Then the substance under experiment is raised to a certain temperature and placed in the test tube; it melts a certain quantity of ice, and thereby causes a diminution in volume of the contents of ft, and consequently a fall of the mercury in c, and also in the graduated tube d. In this way the weight of ice melted may be estimated, and the weight and temperature of the tested substance being known, the specific heat may be readily calculated according to the formula which has been given. 2. The method of mixtures. An outline of this method was given in defining specific heat; it will now be applied in making determinations. A body is weighed and raised to a certain temperature, and then placed in a vessel containing cold water whose weight and temperature are also known. Let m be the weight of the body, n its temperature, and s its specific heat; also let w be the weight of cold water, and t its temperature. After a time equilibrium is obtained, when the temperature may be represented by e. The quantity of heat which the body has lost will therefore he m s (n - e), and that which has been gained by the water will be w (e - t), the specific heat of water being unity.

Now, as the quantity of heat which is absorbed is equal to that which is given out by the body under experiment, m s (n - e) = w (e - t), from whence s = w (e-t)/m(n-e) . To apply this formula, suppose that three ounces of mercury at 212° is mixed with one ounce of water at 32°, and that the resulting temperature is 48.2°, what is the specific heat of mercury? In this example m = 3, e = 48.2°, and n - e = 163.8°; therefore s = 48.2-32/ 3 x 163.8 =16.2/491.4 = .033. the specific heat of mercury, which is therefore only about 1/30 that of water. In accurate experiments corrections are required for errors, one of which is caused by the absorption of a small amount of heat by the containing vessel. Regnault devised a method of mixtures, using a calorimeter capable of yielding more accurate results, and the elaborate experiments which were made by him have been of great value in the arts; but the method given above sufficiently illustrates the principles involved. 3. The method of cooling. Equal weights of bodies having different specific heats will cool through different degrees of temperature in the same time, the body having the least specific heat cooling the most rapidly.

If two thermometers with blackened bulbs and of the same size are filled, one with mercury and the other with water, and then, at a common temperature, are placed in cool enclosures of the same construction and temperature, the mercurial thermometer will cool more than twice as rapidly as the one of water, the proportion being 30 to 13, because the specific heat of water is 30 times that of mercury, while the specific gravity of mercury is 13 times that of water. - Specif c Heat of Solids. It was found by Dulong and Petit that the specific heat of a solid is greater at a high than at a low temperature. Their results are shown in the following table:

Diseases Of The Heart 0800391

Fig. 8.

Black's Ice block Calorimeter.

Fig. 9. - Black's Ice-block Calorimeter.

Ice Calorimeter.

Fig. 10. - Ice Calorimeter.

Bun sen's Calorimeter.

Fig. 11. - Bun-sen's Calorimeter.

MEAN SPECIFIC HEAT OF SOLIDS.

SUBSTANCES.

Between 32° and 212° F.

Between 32° and 572° F.

Iron...

0.1098

0.1218

Mercury........................

0.0330

0.0350

Zinc........

0.0927

0.1015

Antimony.......

0.0507

0.0549

Silver........

0.0557

0.0611

Copper........

0.0949

0.1013

Platinum.......

0.0355

0.0355

Glass.......

0.1770

0.1990

In the above table it may be seen that the specific heat of all the substances is greater at high than at low temperature, except that of platinum, which remains the same within the limits of the experiment. The reason given for this is that the melting point of platinum is very high, far higher than that of cast iron, and Regnault has found that the increase in its specific heat becomes more rapid as it approaches its melting point. Pouillet, by the method of mixtures, obtained the specific heat of platinum at higher temperatures than those employed by Dulong and Petit, but still very far below the melting point. The following are his results, which differ somewhat from those of Dulong and Petit:

Mean Specific Heat Of Platinum

Between 32° and 212° F....................... 00335

" 82 " 572 ...................... 0.0343

" 82 " 932 ...................... 00352

" 32 " 1292 ......................0.0360

32 " 1832 ...................... 0.0373

32 " 2192 ...................... 0.0382

Contrary to the results of Dulong and Petit, Pouillet found there was a variation between 32° and 572°, but it will be seen that they agree as to the increase of specific heat with increase of temperature. The specific heat of a solid depends upon its molecular conditions, which may be considerably changed by treatment, as by rate of cooling after fusion, by hammering, by compression, or by traction. An increase of density diminishes the specific heat, while expansion increases it; for which reason, probably, it increases with the temperature. The following table of specific heats of solids is by Regnault, the range being between 32° and 212° F.:

Substances.

Sp. heats.

Animal charcoal.....

0.26085

Wood charcoal.......

0.24111

Sulphur..........

0.20259

Graphite............

0-20187

Glass.............

0.19768

Phosphorus.....

0.18949

Diamond.......

014GS7

Gray iron.......

0.12983

Steel................

0.11750

1ron................

0.11397

Nickel..............

0.10863

Substances.

Sp. heats.

Cobalt..............

0.10696

Zinc............

0.09555

< topper.............

0.09515

Brass...........

0.09891

Silver...............

0.05710

Tin............

0.05623

Antimony.......

0.05077

Mercury.....

008882

Gold..............

0.03244

Platinum........

0.03244

Bismuth.......

0.03084

Specific Heat Of Liquids

The specific heat of liquids may be found by the method of cooling, by that of mixtures, or by the calorimeter of Lavoisier and Laplace, fig. 10 already described. Regnault employed the following method : The liquid under experiment is placed in the reservoir a, fig. 12, and this is immersed in a vessel containing water at a certain temperature; a known temperature is therefore given to the liquid in the reservoir by agitating the water in the bath. The stopcock d is then opened, and the fluid is forced into the vessel e, contained in the calorimeter c. The water in the calorimeter, which is cooler than the fluid under experiment, has its temperature raised by the introduction of the latter. The increase is measured by the thermometer t, and from this, the weight of the water in the calorimeter and of the fluid under experiment being known, the specific heat of the latter is determined according to the method given above. Generally, a substance has a greater specific heat when in a liquid than when in a solid state, a fact first observed by Irvine. Thus, the specific heat of ice is only half that of water. The specific heat of liquids also increases with the temperature, but in a greater ratio than that of solids.

The following results were obtained by Regnault with water:

Regnault's Method for Liquids.

Fig. 12. - Regnault's Method for Liquids.

Mean Specific Heat Of Water

From 32° to 104° F............................ 1 .0013

" 32 " 176 ............................ 1.0035

" 32 " 248 ........................... 1.0067

" 32 " 320 ........................... 1.0109

" 32 " 392 ............................ 1.0160

" 32 " 446 ............................ 1.0204

It was formerly thought that water had a greater specific heat than any other liquid, but the researches of Dupre. and Page indicate that the specific heat of a mixture of water and alcohol, containing 20 per cent. of the latter, is probably as high as 1-05. - Specific Heat of Gases. The specific heat of a gas at a constant volume differs from that at a constant pressure; in other words, it takes a greater amount of heat to raise a certain quantity of a gas through a certain number of degrees of temperature if it is allowed to expand than when it is confined. The specific heat under constant pressure exceeds that of constant volume by the amount which would be consumed in producing the expansion. The first important researches on the specific heat of gases were those of Delaroche and Berard. Their method consisted in passing known volumes of a gas under constant pressure and temperature through a spiral tube immersed in water, and making their calculations from the increase in its temperature. Re-gnault afterward made more exact experiments with a modification of the apparatus, from which he arrived at the following conclusions: 1. The specific heat of a given weight of a gas which is approximately perfect, or non-conden-sible, does not vary with the temperature of the gas. 2. The specific heat of a given weight of such a gas does not vary with the pressure or density, and therefore the specific heat of a given volume does vary in proportion to the density. 3. The specific heats of equal volumes of simple and uncondensible gases and of compound gases which are formed without condensation, such as hydrochloric acid and nitric oxide, are equal. 4. These laws do not hold for condensible gases, either simple or compound, as chlorine, bromine, or carbonic acid gas, the specific heat of which increases with the temperature. - Specific Heat of Atoms. Before treating of latent heat it will be convenient to consider the law of atomic heat, or the specific heat of atoms, which was discovered by Du-long and Petit in 1819, and which has rendered the knowledge of the specific heats of bodies of so much importance in chemical investigations.

This law may be exactly enunciated as follows : The specific heats of elementary bodies are inversely proportional to their atomic weights; in other words, the product of the specific heat of any element into its atomic weight is constant. The following results verifying this law are due to Regnault; only a par-tial list is given :

ELEMENTS.

Sp. heat.

Atomic weight.

Product, or sp. heat of atoms.

Sulphur....

0.1776

32

5.6832

Magnesium...

0.2499

24

5.9976

Aluminum...

0.2143

27.5

5.8932

Zinc....

0.0955

65

6.2075

Cadmium ..................

0.0576

112

6.3594

Cobalt......................

0.1070

58.5

6.2595

Nickel.....................

0.1091

58.5

6.3823

Iron.......................

0.1138

56

6.3728

Manganese..................

01140

55

6.2700

Copper.....

0.0951

63.5

6.0389

Silver....

0.0570

108

6.1560

Gold.......................

0.0324

196

6.3504

Antimony..................

0.0508

122

6.1976

Bismuth........

0.0308

210

6.4680

Potassium..................

0.1696

39

6.6144

Sodium.....

0.2934

23

6.7482

Lithium....................

0.9408

7

6.5856

Lead.....

00814

207

6.4998

Platinum...................

0.0324

197

6.3828

Arsenic........

0.0814

75

6.1050

Iodine..........

0.0541

127

6.8707

Bromine (solid).............

0.0843

80

6.7740

Mercury (solid).............

0.0319

200

6.3800

It will be observed that the products are not exactly the same, but there are the strongest reasons for believing that the variations are owing to differences in physical condition which are unavoidable under the circumstances in which the experiments are made. Assuming the theory to be correct, it follows that all elementary atoms, independent of their weight, have the same specific heat, and therefore that masses of elementary substances containing the same number of atoms and under the same physical conditions require the same amount of heat to raise them through an equal number of degrees. Thus, the atomic weight of iron being 56, and that of mercury 200, it will require the same amount of heat to raise 56 pounds of iron or 200 pounds of mercury through the same number of degrees. Neumann and Regnault have also found that the specific heats of all compound bodies of similar atomic composition are inversely proportional to their atomic weights. The following are Regnault's results with bichlorides:

SUBSTANCES.

Sp. heat.

At. weight.

Product.

Chloride of barium, BaCl2...

0.0896

208

18.64

" strontium, SrCl2.

0.1199

158.5

19.00

calcium, CaCl2...

0.1642

111

18.23

" magnes'm, MgCl2

0.1946

95

18.49

lead, PbCl2......

0.0664

2T8

18.46

" mercury, HgCl2..

0.0689

271

18.67

" zinc, ZnCl2......

0.1362

186

I8.52

tin, SnCl2.......

0.1016

1S9

19.20

The following results were obtained with car bonates:

SUBSTANCES.

Sp. heat.

At. weight.

Product.

Carbonate of lime, CaCO3........

0.2086

100

20.86

" barytes, BaCO3..

0.1104

197

21.75

" strontium, SrCO3.

0.1448

147 5

21.36

" iron, FCO3,

0.1934

116

22.43

It will be seen that the numbers in each table agree together more nearly than those of one with the other, but the close agreement in each group justifies the adoption of the law. IV. Latent Heat. The doctrine of latent heat was taught by Black in 1702. He was the first to observe that when a body passes from a solid to a liquid state a quantity of heat disappears. Thus, if ice at 32° has heat applied to it, and the resulting water as well as the ice is stirred, the temperature will remain at 32° until all the ice is melted. Thus, all the heat which has during this time been absorbed will have disappeared, and was said by Black and his contemporaries to have become latent. According to modern theory, this is not strictly true, unless we consider its conversion into another force a latent power which may bo again reconverted into heat by the reconversion of the water into ice. The energy which manifests itself in heat vibrations is expended in maintaining a different form, or performing a certain amount of internal work, as it is called. - Latent Heat of Fusion. If a pound of water at 212° is mixed with a pound of water at 32°, the resulting temperature will bo a mean, viz., 122°; but if a pound of ice at 32° is mixed with a pound of water at 212°, the result will be two pounds of water at 51°.

There is thus a difference in the heat of the two mixtures of 71°, and since the temperature of one of the constituents in each mixture, viz., boiling water, was the same, this difference of 71° must represent the heat which is required to liquefy one pound of ice, and which is the same as that required to raise two pounds of water through a range of 71°, or one pound of water through 142°, or 142 pounds of water through 1°. If we take as a unit of heat that quantity which is necessary to raise one pound of water through 1°, the latent heat of water will be represented by 142 on Fahrenheit's scale, and by 78.88 on the centigrade scale. The experiment may be varied by mingling a pound of ice at 32° with a pound of water at 174°, when the resulting temperature on the fusion of the ice will be found to remain at 32°, showing as before the expenditure of 142°, which is the latent heat of water. According to the experiments of M. Person, the latent heat of water is more nearly 142.65°, or on the centigrade scale 79.25°. The following are his results with other liquids, calling the latent heat of water a thermal unit:

Table Of Latent Heats

substances.

W=l.

In deg. F.

In deg. C.

Water..............

1.000

142.650

79.250

Phosphorus........

0.663

9.061

5.034

Sulphur.....

0.118

16.862

9.368

Nitrate of soda...

0.794

113.355

62-975

Nitrate of potash ...

0.598

85.268

47.371

Tin................

0.179

25.653

14.252

Bismuth...........

0.159

22.752

12.640

Lead ..............

0.067

9.664

5.369

Zinc...............

0.355

52.434

28.130

Cadmium..........

0.172

24.588

13.660

Silver..............

0.266

37.926

21.070

Mercury...........

0.035

5.094

2.830

Latent Heat Of Vaporization

Liquids in passing into a state of vapor absorb a vast amount of heat. The conversion into vapor may be rapid, as in boiling, or it may be slow, as when water evaporates in the open air at common temperatures. In either case disappearance of heat in proportion to the quantity evaporated is the result. If a flask of cold water is placed over a lamp, the temperature will continue to rise until it reaches 212° F., when ebullition will commence; but the temperature will remain at 212° until the water has all boiled away. If the water at the commencement of the operation is at 32°, and the supply of heat is uniform, the time occupied in evaporating it will be about 5 1/2 times that which is occupied in raising it to the boiling point, although the temperature has not risen above 212°; therefore 5 1/2 times as much heat is absorbed in evaporating a given quantity of water as in raising it through 180°. The latent heat of steam is therefore about 5 1/2 times 180°, or 990° F. If the steam is reconverted to the liquid form, precisely this amount of heat reappears; in other words, the energy into which the heat was converted to maintain a state of vapor is reconverted into heat when the steam is reconverted into water.

This is shown in the method of Despretz for determining the latent heat of vapors, which consists in condensing them in a worm immersed in water, and estimating the quantity of heat imparted to the latter. The retort C, fig. 13, heated by a lamp, contains the liquid whose vapor is the subject of experiment. The vapor in passing through the worm S is condensed, imparting its latent heat to the water in the vessel R, and being collected in a vessel placed under the stopcock?', its weight can be found; and that of R, or the calorimeter, and its contents being known, and also their temperature, the increase of the latter furnishes the data for caldilating the latent heat of the vapor. Re-gnault used more elaborate apparatus, and his results were rather more exact. If pressure is applied to a gas confined in an enclosure, its temperature will be raised, and if the pressure is immediately removed, the gas will return to the temperature it had before compression; but if, while under compression, it is allowed to cool to its previous temperature, and the pressure is then removed, it will fall through as many degrees as it had been raised by compression. Upon the principle here involved, gases which were formerly considered permanent have been reduced to a liquid and to a solid condition.

Faraday employed the following method: Introducing materials for producing a gas in one end of a bent tube, fig. 14, and heating it previous to their combination, and then applying a gentle heat, a vast pressure was produced by the generated gases, and then by placing the other end of the tube in a freezing mixture, condensation was effected. Thilorier in 1834 constructed on this principle an apparatus which was capable of liquefying large quantities of carbonic acid gas. The operation requires a pressure of about 50 atmospheres, or about 700 lbs. to the square inch. The vessels were formerly made of cast iron, strengthened with wroughtiron hoops; but explosions occurring, attended with loss of life, the construction was modified by using leaden vessels surrounded with copper ones, bound with strong iron hoops. The apparatus is represented in fig. 15, and consists of two vessels, one a condenser and the other a generator, the latter being represented in section. Bicarbonate of soda is placed in the generator, and also a cylindrical vessel containing sulphuric acid. The generator being supported by pivots, it can be turned to spill the acid.

The resulting gas, evolved in large quantities, is forced through the connecting tube into the condenser, which is surrounded by a freezing mixture, and is there condensed into a liquid. When some of the liquefied gas is allowed to escape into the air, a portion expands into gas, which so chills the remainder that it solidifies and forms white flakes, like snow, its temperature being about - 129° F.; and if this is mixed with ether, the cold which is produced is so intense as to have an effect upon the skin like that of burning with hot iron. By placing this mixture in the exhausted receiver of an air pump, Faraday caused the temperature to fall to 166° below zero; and M. Natterer by the use of a bath of nitrous oxide and bisulphide of carbon, previously liquefied by cold and pressure, lowered the temperature to 220° below zero; and Despretz succeeded in reducing alcohol to a viscous state. Liquid carbonic acid contained in a tube and placed in this mixture instantly becomes solid, assuming the appearance of transparent ice. By the use of this mixture and very high pressure, Andrews reduced air to 1/675 of its original volume, oxygen to 1/554, hydrogen to 1/500, carbonic oxide to 1/278, and nitric acid to 1/680, but without producing liquefaction.

There was some departure from Ma-riotte's and Boyle's law (see Atmosphere), but it was less in hydrogen and carbonic oxide than in the other gases. Freezing on a large scale by Carre's apparatus, described in the article Freezing, is effected on the principle of absorption of heat by evaporation and expansion. The absorption of heat by liquefaction has a familiar example in the ordinary freezing mixture of snow or pounded ice and common salt, by which the zero temperature of Fahrenheit's thermometer was obtained. - An interesting experiment in the absorption of heat by liquefaction and its reappearance on solidification is made by dissolving sulphate of soda in water. The two being mingled at the same temperature, the thermometer will indicate a fall. If the solution is warmed and saturated, and then allowed to cool while perfectly at rest, a point will be reached at which more of the soda will remain in solution than could have been dissolved at the same temperature. The polar relations of the molecules of the salt by which solution is maintained require, in order that solidification may take place, to be disturbed by a further reduction of temperature or by a mechanical impulse.

The condition of solution is maintained by an expenditure of energy which when solidification or crystallization takes place resumes the condition or motion of heat. Agitation of the vessel, or of its contents by dropping among them a crystal of the salt, will cause crystallization to commence; and the bulb of a thermometer plunged into the mass will show a rise of temperature. - Prof. James Thomson, in a paper published in the "Transactions of the Royal Society of Edinburgh" in 1849, expressed his opinion, deduced from the mechanical theory of heat, that a liquid which expands in solidifying, like water, must have its melting point lowered by increase of pressure. Sir William Thomson soon after tested the question by experiment, and proved the correctness of the deduction. When a mixture of ice and water was subjected to pressure, the temperature fell, returning again to 32° when the pressure was removed. The addition of pressures of 8.1 and 10.8 atmospheres lowered the freezing point 0.106° and 0.232° F. respectively; results which very nearly agree with Prof. James Thomson's prediction that the fall should he 0.0135° for each additional atmosphere. Mouson has since then succeeded by enormous pressure in reducing the freezing point of water several degrees.

The apparatus in which pressure was effected was placed in a certain position and. charged with water into which a piece of metal was dropped. The water was then frozen, and cooled to zero, or 32° below the freezing point. A pressure which was estimated to he several thousand atmospheres was then applied, after which the apparatus was inverted and. the pressure removed, when on examination the piece of metal was found at the opposite side of the enclosure, thus showing that the ice had been melted. Those bodies which, unlike ice, expand during liquefaction, have their melting points raised instead of lowered by increase of pressure. In this manner Bunsen, Hopkins, and Fairbairn have raised the melting point of spermaceti, which is 120°, several degrees; a pressure of 519 atmospheres raised it to 140°, and one of 792 atmospheres to 176°. A liquid which, like water, expands on congealing, has its particles restrained by pres-sure, and therefore to congeal it the temperature must be lowered; but one which contracts in solidifying will have its particles assisted by pressure, and hence its melting point will be raised. - Many interesting phenomena are exhibited by liquids and gases when subjected to great heat and pressure, such as the obliteration of the line of demarkation between the liquid and vaporized portion in which what is called a critical temperature is concerned.

The subject will be found treated, with that of the tension of vapors, in the article Vaporization. Chemical action being always accompanied by physical change, as expansion or contraction, liquefaction or solidification, it is difficult to estimate the effects produced by each. In general it may be held that the heat of chemical combination results from the intense molecular motion imparted by the clashing of combining molecules with each other, and moreover, that whatever heat is evolved by combination will be absorbed, or will disappear in the separation of the constituents of the compound into their original form; and it is found that generally combination produces heat, and that decomposition produces cold. But the heat which is evolved by the physical changes which accompany chemical action is more easily accounted for. Take for example the condensation which accompanies the union of quicklime with water; the resulting hydrate has less bulk than the sum of the constituents previous to combination.

The energy necessary to maintain this excess of volume among particles at insensible distances from each other composing liquids or solids, is enormous; consequently a reduction of the distances, whether accomplished by the influence of chemical affinity or by mechanical pressure, causes a conversion of this energy into another, generally heat. The first change may not, however, be entirely into heat, but, as in the case of the compression of certain crystals, or the combination of a metal with an acid under certain conditions, as in the galvanic battery, there may be a transformation into electric force, but which is supposed finally to become resolved into heat. Sir William Thomson has advanced the opinion that there is a tendency to the conversion of all physical energy into the condition of heat, and to its uniform diffusion throughout all matter; a condition which ho regards as involving the cessation of all physical phenomena. The conclusions of Prof. Thomson are founded upon the law of the French philosopher Carnot, which is that mechanical energy is produced by heat only when it is transferred from a body of a higher to one of a lower temperature.

The subject is a difficult one, as there are many possible circumstances connected with the forces and matter of the universe which can never be reduced to an exact basis of calculation. - The following are the most important recent works on heat: "Sketch of Ther-mo-dynamics," by P. G. Tait (Edinburgh, 1868); "An Elementary Treatise on Heat," by Balfour Stewart (London, 1872); "Theory of Heat," by J. Clerk Maxwell (1872); "Heat as a Mode of Motion," and " Contributions to Molecular Physics in the Domain of Radiant Heat," by John Tyndall (1873). See also the articles "Heat" and "Radiation" in Watts's "Dictionary of Chemistry," and various articles in the reports of the Smithsonian institution.

Despretz's Apparatus for Latent Heat of Vaporization.

Fig. 13. - Despretz's Apparatus for Latent Heat of Vaporization.

Faraday's Apparatus for Liquefying Gases.

Fig. 14. - Faraday's Apparatus for Liquefying Gases.

Thilorier's Apparatus for Liquefying Carbonic Acid.

FIG. 15. - Thilorier's Apparatus for Liquefying Carbonic Acid.