Chemistry, the science which investigates the composition and certain properties of material substances. Nothing is certainly known of the derivation of the term, but it is most probably fromChemistry 0400157 (Chemia), the original name of Egypt, in which country it is supposed to have originated; hence it was called the Egyptian art. Others derive it fromChemistry 0400158 or Chemistry 0400159 relating to juices; hence the word was formerly written chymistry. The oldest au-thor who mentions it is Julius Maternus Eir-micus, in the reign of Constantine, about A. D. 340. Suidas defines chemistry as the making of gold and silver; Libavius, in 1505, as the art of making chemical preparations and of extracting the pure essences in a separate form from mixtures; Lemery, in 1675, as the art which treats of separating the different substances which occur in mixtures; Bergman, in the latter part of the 18th century, as the science which investigates the components of bodies in regard to their nature, their properties, and the manner in which they are combined; Macquer, about the same time, as the science which makes known to us the nature and properties of all bodies by composing and decomposing them. To the last two definitions, which express quite fully the ideas of chemists of the present day, may be added that of Berzelius: "Nature is composed of certain elementary bodies or elements.

The knowledge of these bodies, of their mutual combinations, of the forces by which these combinations are brought about, and of the laws in accordance with which these forces act, constitutes chemistry." - Chemistry is often incorrectly spoken of as a science of recenl origin. This view may be readily disproved. It would be impossible to determine at what exact period it became so far developed as to deserve the name of science. Many chemical facts must have been known from the earliest times. As far back as history goes they are treated of. Only by the gradual collection and explanation of such facts could chemistry become a science. The Egyptians possessed considerable chemical knowledge. The smelting of ores and working of metals must have been brought to a high degree of perfection among them; otherwise many of their works of art, still existing, could not have been made. Their skill in dyeing implies a knowledge of the necessary chemicals and mordants. They also made glass, even colored, and knew how to prevent the decomposition of dead animal matter. The priesthood evidently possessed more or less knowledge of pharmaceutical chemistry. The Phoenicians dyed, made glass, and imported tin.

The Israelites seem to have obtained some chemical knowledge, especially relating to metallurgy, during their sojourn in Egypt; they were acquainted with gold, silver, copper, tin, lead, and iron. The Greeks in the time of Homer seem to have had only what chemical knowledge they had derived from the Egyptians and Phoenicians. The metaphysical direction of the Greek mind was not calculated to advance the science by experimental investigations. They attempted, on the other hand, to explain the composition and origin of matter by philosophical speculations, as seen in the discussions regarding the elements, some five or six centuries before Christ. The views advanced by Aristotle on this subject long exerted a great influence on the science. He considered substances to be the result of the mixture of several fundamental properties, and their components or elements only as bearers of these properties, not at all as material, undecomposable substances which could be extracted experimentally. According to Aristotle these properties are those evident to the senses, viz.: hot, cold, dry, wet, heavy, light, hard, soft, etc. He recognized only the first tour of these, because the others are less general and for the most part only secondary results of the union of the preceding.

Hence his conclusion that there are four fundamental properties, hot, cold, dry, and wet, which characterize the four elementary conditions of matter, or elements, fire, air, earth, and water, of which all substances are composed, and from which their peculiarities are derived. He admits that each element possesses two of these properties at the same time, and since no element can have at once two totally opposite properties, cannot be simultaneously wet and dry, only four combinations are possible. Thus, simultaneous dryness and heat form fire; heat and moisture, air; cold and dryness, earth; cold and moisture, water. The Romans derived some chemical knowledge from other nations. They were acquainted with the metals previously mentioned, also with mercury, the power of which to dissolve gold they made use of in gilding. They knew of various alloys, of steel, glass, vinegar, and soap. Their knowledge of dyeing was less than that possessed by the Egyptians. - Although but little is known of the progress of chemistry from the 1st to the 4th century A. D., it is probable that much of the knowledge which had long been kept secret by the Egyptian priests was now published and taught at Alexandria. Previous to the latter period chemistry had consisted only of a collection of empirical facts without intimate connection or definite purpose, upon which no theoretical views of any kind had been founded.

It has been a peculiarity of chemistry, that while preserving its name and connection as a science it has had temporarily, at various times, special objects in view, to the attainment of which all its energies were directed for the time, while what is now regarded as the true aim of the science was used only as a means of obtaining the desired result. Such a period now commenced. It was characterized by the purpose which the science served, as well as by a theory upon the composition of the metals. All chemical facts were now studied under a certain connection for the purpose of solving a given problem: how to convert base metals into noble, as iron or lead into silver or gold. This idea is first mentioned by Greek authors of the 4th century, who speak of it as a thing already well known. It probably originated with the Egyptians, from whom it passed to the Arabs when they overran Egypt in the 7th century. From the middle of the 8th century the Arabs paid much attention to chemistry. From them its study passed through Spain into northern Europe. The purpose was still the transmutation of metals, the discovery of the philosopher's stone, the touch of which would convert mercury into gold, and at a later period also regarded as capable of curing all diseases.

The first chemical theory appears in the 8th century. It consisted essentially in this: All metals are composed of two constituents, upon the relative quantity and purity of which the nature of the metal depends. These components were called sulphur and mercury. Although not possessing all the properties of ordinary sulphur and mercury, they derived their names from these last, which were supposed to contain a large proportion of them, but their properties were nevertheless different. The sulphur and mercury of the alchemists must be regarded, therefore, as conventional elements, deriving their names from certain analogies which they bore to ordinary sulphur and mercury. By mercury, undecomposability appears to have been understood; it was also regarded as the cause of metallic lustre and of malleability; by sulphur, on the contrary, de-composability was understood; these terms referring to the behavior of substances when subjected to fire, at that time regarded as the most powerful chemical agent.

The various metals were supposed to be composed of these two bodies in different quantities, degrees of purity and of fixity (affinity?). The fusibility of the metals was considered to depend upon the fixity of these ingredients; their color upon the relative amount of sulphur which they contained. The author of this theory is unknown. It is fully developed in the writings of the Arab Geber, who refers it to the ancients. It experienced but slight alteration till toward the end of the alchemical period, when the idea of salt was added to those of mercury and sulphur. Among the Arabs, physicians were the principal chemists, many of them being highly scientific men. The writings of Geber, in the latter half of the 8th century, indicate the amount of chemical knowledge which they then possessed. He describes the metals very accurately, and mentions the different degrees of affinity which mercury has for gold, silver, lead, tin, and copper, He knew how to convert metals into oxides by means of heat, and how to purify native sulphur by solution in alkaline lye, and reprecipitation by vinegar. Several metallic sulphides were also known to him, as well as the fact that metals increase in weight and change color when heated with sulphur.

The Arabs, unlike some of their predecessors, did not consider simple change of color in a metal to be transmutation. They demanded an entire change of its properties - that it should be made like some other metal. Transmutation was regarded by them in the light of a scientific problem as yet unsolved. Their knowledge of salts was comparatively extensive, alum, saltpetre, sal ammoniac, and green vitriol being accurately described. The carbonates of the fixed alkalies, and the use of lime to render them caustic; the preparation of sulphuric acid by the distillation of alum; of nitric acid by the distillation of saltpetre and green vitriol; the preparation of strong acetic acid from vinegar, and ofaqtta regia from nitric acid and sal ammoniac, are all described by Geber. By means of the acids thus obtained, artificial salts were prepared, as nitrate of silver and bichloride of mercury; gold was also dissolved. They purified their preparations by distillation, by recrystallization, and by sublimation. The purification of the noble metals by cupellation, the use of the water bath, and processes of filtration, were also known. Several words still in common use, as alkali, alcohol, etc, originated with them.

Geber himself published a special work on the construction of chemical furnaces. - The importance of the Arabs as chemists ceased with the 12th century: not, however, before they had awakened in other nations an enthusiasm for the science. During the 13th century it had spread over the greater part of northwestern Europe. The views of Albertus Magnus, who flourished in Germany during the 13th century, will serve as an example of those of his day. Believing fully in the transmutation of metals, he considered it easier to eonvert them into each other when their properties are analogous. Tims gold can be more readily made from silver than from any other metal, for the mixture of which it is composed is very similar to that which forms silver. Indeed, it would only be necessary to change the color and weight of the latter, in order to obtain gold. He admits that different species of bodies cannot be converted one into the other. The metals, however, he regarded as mere varieties of one and the same species, being all composed alike.

His views concerning this composition are essentially the same as those of Geber, but he believed that, besides sulphur and mercury, the metals must contain water, to the cold of which their solidity is due, seemingly mingling a portion of Aristotle's doctrine with that of Geber. The knowledge of practical chemistry detailed in his works is in advance of that possessed by Geber. For example, he describes the separation of silver from gold by means of nitric acid; a method of preparing metallic arsenic; and observed that sulphur attacks all metals, except gold, when heated with them. Contemporary with Albertus lived Roger Bacon, a most able diffuser and promoter of chemical science. At the same time, Raymond Lully made many new and important observations, but introduced an obscure style of writing, and fell into many absurdities, like those which soon became characteristic of alchemy. The transmutation of metals was now no longer spoken of as a possibility, but as a well established fact. In the 14th century alchemy had become diffused over the greater part of the civilized world.

It was, however, especially among the priesthood that its followers were to be found, in spite of its prohibition by a bull of Pope John XXII. in 1317. In the latter part of the 15th century flourished Basil Valentine. In addition to the elements, sulphur and mercury, of the earlier alchemists, he mentions salt. From these three substances he supposes that not only metals but all substances are composed. He first clearly described bismuth and zinc; prepared antimony and several new salts; also muriatic acid, by distilling common salt with green vitriol. He knew how to precipitate copper from its solutions by means of iron, also gold by means of mercury, and had much general knowledge of precipitation. With him qualitative analysis first appears. He detected iron in many specimens of hard tin, copper in the brittle iron of Hungary, and silver in the copper from Mans-feld. He explains how a similar occurrence of the noble metals in the base metals of commerce may have given rise to many of the so-called transmutations of false alchemists. - The great impulse given to all branches of learning by the remarkable events of the 15th century had its effect on chemistry.

The overthrow of the Byzantine empire, which scattered many of its learned men over western Europe, the invention of printing, and, above all, the growing tendency of men to think for themselves, were of peculiar importance to it. The implicit confidence hitherto placed in noted authorities began to diminish, and their theories to be questioned. The ground was made ready for a new system. This was inaugurated by Paracelsus, in the first quarter of the 16th century. It fills a well defined period in chemical history, enduring till the middle of the 17th century. The views regarding the transmutation of metals remained unchanged, but gold-making was no longer the chief aim of the leading chemists. Indeed alchemy, as an art, soon became separated from chemistry. The characteristic of this period was the intimate connection between medicine and chemistry, the whole purpose of the latter being to cure disease. As its purpose was thus elevated, it passed into the hands of educated men, through whom the amount of chemical knowledge was rapidly increased. Paracelsus regarded sulphur, mercury, and salt as the elements of all substances, organic and inorganic.

His ideas of sulphur and mercury correspond with those of the earlier alchemists, while that of salt is opposed to that of mercury, the former being typical of solidity and incombustibility, while the latter is expressive of volatility without decomposition. Besides giving a great impulse to the science, Paracelsus introduced numberless chemical preparations into use in medicine, rendering them familiar to physicians and apothecaries, whence the latter were often induced to occupy themselves with the study of chemistry. The controversies between the followers of Paracelsus and his adversaries also excited great interest in the science. At this epoch Agricola (1490-1555) flourished in Saxony. Standing aloof from the great chemical questions of the day, he occupied himself almost exclusively with metallurgy, in which specialty he became very distinguished by his writings. He first describes clear processes of smelting metals and of assaying ores. He may also be regarded as the founder of this branch of chemical science. Distinguished among the opposers of the mistical style of the alchemists, and for his exposition of their charlatanry, was Libavius (died 1616). Many valuable observations are also due to him.

He prepared sulphuric acid by burning sulphur with salpetre, and proved its identity with the acid obtained by roasting green vitriol or alum. By distilling tin with corrosive sublimate he prepared bichloride of tin, still known as the fuming liquor of Libavius. The ideas concerning elements entertained by Van Helmont (1577-1644) differ essentially from those of preceding chemists. He rejected the four elements of Aristotle, for fire is not a substance; heat and cold are only abstract qualities, and not material things; therefore fire cannot be contained in any substance as a material component. He denounced, on the other hand, the elements of the alchemists, sulphur, mercury, and salt, and especially the theory that they were elements of the animal body, in proof of which, according to him, no facts existed. He regarded water as the chief ingredient of all things. It could be obtained by burning any combustible body. From it all parts of vegetables are formed, their earthy as well as combustible portions. In proof of this, Van Helmont made the following experiment: A willow twig weighing 5 lbs. was planted in a pot containing a known quantity of thoroughly dried earth; this was covered to protect it from dust, and watered daily with rain water during five years.

The willow, meanwhile, grew large and strong, weighing 164 lbs., although the earth in the pot, when again dried, had only lost 2 oz. This was regarded as proof positive that water alone forms plants and the mineral matters contained in them, while animals obtain their components directly from plants. Van Helmont introduced the term gas into chemistry, described several different kinds of gases, and distinguished them from vapors. As the practical chemist of this period, Glauber (1G04-'68) stands preeminent. By improving the processes of preparing the mineral acids, by the discovery of salts, and by observing many new facts, he did great service. By synthesis he obtained a knowledge of the composition of many substances. It is worthy of remark that, during the latter half of the 17th century, Tache-nius made the first approximately correct quantitative chemical statement which occurs in the history of the science, viz.: that metallic lead when burned to red lead increases its weight by 1/10 In the latter half of the 17th century the foundation of several learned societies promoted and advanced the study of chemistry as well as of the other sciences. - During the continuance of this period of intimate connection between chemistry and medicine a great amount of chemical knowledge had been collected; the metals and their compounds were well known, as were the three principal mineral acids and their combinations with the alkalies.

Toward its close a mass of observations, the material with which the structure of modern chemistry has been reared, invited explanation and classification. Important views were at this period advanced by Boyle (1 626-'91). He first treated the question of elements from the same point of view as has been taken by modern chemists. He proved more clearly than any of his predecessors that the four elements of Aristotle were inadmissible, and how little the elements of the alchemists were calculated to afford a rational conception of the composition of bodies. He thought that, rather than seek to explain the primary elements of matter, which admit of various views, attention ought to be specially directed to those ingredients which can be separated and isolated as such. If these cannot be further decomposed, they should be called elements, although he admits that they may be still further decomposed as knowledge increases. This clear definition was in his eyes much better calculated to advance the progress of chemical knowledge than the vague ideas expressed in the theories of Aristotle and the alchemists. Boyle's views, however, were not admitted by his contemporaries.

He also first defined acids and alkalies in reference to their action on vegetable colors, and showed that what has been dissolved in the one may he precipitated by adding the other. In applying this to analysis, he made a great step, the so-called dry method of analysis having been previously exclusively used. To this day several reactions first described by Boyle are in common use; for example, the detection of ammonia by adding lime to the matter containing it, and observing the fumes that are formed in presence of an acid; also the detection of silver by muriatic acid. He likewise made many observations of affinity. Nicholas Lemery (1645-1715), a famous lecturer at Paris, deserves mention for his efforts in the diffusion of chemical knowledge. As a clear and systematic writer, he was justly celebrated. - Another modification of views regarding the elements, which exerted great influence, was expressed by Becher (died 1682). According to him, all inorganic bodies are composed of earthy ingredients. There are three elementary earths, the fusible, the combustible, and the mercurial, severally principles of fluidity, combustibility, and volatility. These three earths are present in all metals, combined in different proportions.

When combined with water they form the salts, and also a universal acid which is the basis of all acids. He regarded the calcination (oxidation) of metals and combustion in general as a process of decomposition depending on the expulsion of the combustible earth by means of fire. A simple body incapable of decomposition could not burn, for every body capable of burning must contain within itself a cause of its combustibility. This doctrine was soon adopted by Stahl, and under the name of the phlogiston theory characterized an epoch in the history of the science. Chemistry now first stands out on an equality with the other natural sciences. Its aim is no longer either the making of gold or the curing of disease. A desire to acquire a knowledge of the composition of bodies, to explain the phenomena which accompany their formation and decomposition, and to ascertain what relation exists between their properties and their composition, became now the motive of chemists. Modern chemistry, properly so called, now commenced. The first special problem which attracted attention was the explanation of the phenomena of combustion and oxidation. The analogy between these processes had been long observed and often considered.

The ancients regarded combustion as dependent upon the separation of fire, apparently believing the latter to be something material. This view was admitted for a long period. The alchemists expressed a similar idea by their figurative sulphur, which was supposed to be expelled when a body burned. For the term sulphur, Becher substituted combustible earth, and defined more clearly the idea that it was separated during combustion. The notion that combustion destroys was undoubted; something was separated, causing the appearance of flame, while the incombustible residue was one of the components of the compound which had been destroyed. In like manner, when a metal was oxidized, the calx (oxide) was considered to be an educt from the metal. The famous phlogiston theory of Stahl (1660-1734) was a more accurate expression of these views. According to him, all combustible bodies must contain one and the same ingredient, to which they owe their common property, combustibility. This combustible matter he calls phlogiston; its existence, although entirely hypothetical, was regarded by Stahl as being so certain that it was hardly worth while to isolate it, and but few attempts to do so were made by his immediate followers. Later it was thought to be identical with hydrogen.

Expressed in the language of the present day, phlogiston may he regarded as the opposite of oxygen. What is now deemed a combination with oxygen was considered by Stahl to be the result of a separation of phlogiston. During combustion phlogiston is expelled, while the other constituents of the compound remain. Charcoal, which leaves little or no residue when burned, was thought to be nearly pure phlogiston. In general, the combustibility of bodies was supposed to depend on the proportion of phlogiston which they contain. On examining the residues left by different substances after their phlogiston had been expelled, it was thought that a knowledge of their original constitution could be obtained. Experience was thus supposed to teach that phosphorus is a compound of phosphoric acid and phlogiston; that the metals are composed of calxes and phlogiston. If bodies from which the phlogiston has been expelled are heated with others rich in phlogiston, the latter give back phlogiston to the former and the original compound is produced. Thus, when a metal calx is heated with charcoal, the original metal is formed. Besides combustibility, Stahl referred color, solubility in acids, and other chemical properties of bodies, to the amount of phlogiston contained in them.

For example, he showed that metals which had been deprived of their phlogiston could no longer unite with sulphur. These ideas were greatly extended by his followers. Stahl's observations of new facts and upon affinity were numerous and valuable. Although unable to free himself entirely from the vague speculations of the alchemists, he adopted in general the idea of elements suggested by Boyle, regarding as such all peculiar constituents of matter which by uniting with each other and with phlogiston form compound bodies. At this period investigations to ascertain what substances should be regarded as elementary commenced. Thus the metal calxes (oxides) were considered to he elements, as were sulphuric and phosphoric acids, etc. Boerhaave (1668-1738) and Homberg (1652-1715) were especially diffusers of chemical knowlege. The Elementa Chemim of the former, published in 1724, exerted an influence which has probably never been surpassed. His experiments made in order to disprove certain statements of the alchemists deserve mention. It had been stated that mercury could be converted into an infusible metal when subjected to the continued action of heat. Boerhaave maintained a quantity of mercury at a moderate heat during 15 years without effecting such transformation.

Another portion of mercury, being strongly heated in a closed vessel for six months, remained unaltered. It had also been stated by the alchemists that mercury could be converted by repeated distillation into a more volatile body. Boerhaave distilled pure mercury 500 times without perceiving any alteration in its boiling point. From these experiments he inferred that the given statements were erroneous. Geoffroy (1072-1731) published systematic tables of affinity, which exerted a long-continued influence on the science. Marggraf of Berlin (1709-'82) was distinguished as an analyst and technical chemist; he first called attention to the existence of sugar in the beet root and other plants indigenous to Europe. Macquer of Paris (1718-84) pointed out the existence of arsenic acid, and made other new observations. His idea of four material elements, earth, air, tire, and water, illustrates the tendency of the period gradually to do away with the vague expressions of the alchemists. Equally characteristic is the fact that although Macquer had himself often used the quantitative method of analysis in investigating mineral waters and metallurgical matters, and considered it very necessary in such connection, he utterly neglected it when studying theoretical questions, and viewed with indifference the antiphlogistic theory, founded on such unimportant data, as he thought, as mere differences of weight.

He regarded phlogiston as identical with light, and capable of passing through transparent media, thus explaining the reduction of peroxide of mercury by heat, which had been observed in glass vessels. The English chemists of this period were, meanwhile, observing facts which afterward constituted the most effective weapons of the opponents of the phlogiston theory. The discoveries of Black, Cavendish, and Priestley led naturally to its overthrow. The investigation of the alkalies by Black of Edinburgh (1728 '99) was of great importance. It had previously been supposed that when limestone was burned it united with fire and thus obtained caustic properties. From caustic lime this tire could be transferred to the mild (carbonated) alkalies, rendering them caustic, while the lime having given up its fire became again mild. Black disproved this view by showing that the mild alkalies are compound bodies and become caustic, not by combining with fire, but from losing one of their constituents, a gas, which he called fixed air. He proved that limestone loses weight when burned to caustic lime, from which he inferred that the latter is contained in the former. He observed that the mild alkalies effervesce with acids, a gas similar to that given off by burning limestone being evolved.

This gas (carbonic acid) he regarded as the second constituent of the limestone or other mild alkali, drawing the conclusion that the true caustic alkalies must be simple bodies which become mild only by uniting with fixed air. This view, after some opposition, was soon universally received by chemists. Black also distinguished more clearly than had been done before the difference between lime and magnesia. The discovery of latent heat is also due to him. The celebrated physicist Cavendish (1731-1810) added greatly to chemical knowledge by his accurate experiments upon gases. He investigated hydrogen, and was led to consider it identical with phlogiston. According to him, when dilute sulphuric acid is added to metallic iron or zinc, infiammable air or phlogiston separates unchanged from its combination with the calx, while the latter unites with the acid. If concentrated sulphuric acid be used, the phlogiston is no longer set free as inflammable air, but combines with a portion of the acid, forming another gas which is not inflammable (sulphurous acid, already called phlogisticated sulphuric acid by Stahl). Cavendish's view of the identity of hydrogen and phlogiston was soon admitted by the supporters of the phlogiston theory, especially after it was found that calxes could be transformed to metals when heated in an atmosphere of hydrogen.

The original idea of phlogiston was thus somewhat modified. With Stahl a dephlogisticated substance meant one which had been oxidized: thus, sulphuric acid was dephlogisticated sulphur; while at the end of the phlogiston period it meant as well a body which had been deprived of hydrogen: thus Cavendish and Priestley call oxygen dephlogisticated water. Cavendish also investigated carbonic acid and the quantitative composition of atmospheric air. The latter he proved to contain oxygen and nitrogen in the same proportions at all seasons of the year and in different localities. He investigated the changes caused in air by combustion, showing that carbonic acid is formed only when the combustible is of animal or vegetable origin. Synthetically, he ascertained the composition of nitric acid by passing a series of electric sparks through air; also that of water. This last was specially important, having been used with great effect by the opponents of the phlogiston theory. Although few men contributed more to the overthrow of this theory than Cavendish himself, yet he to the last remained true to its tenets, having explained all his discoveries in accordance with them.

His reputation has suffered greatly from this, for the chemists who subsequently correctly explained his observations have shared with him the merit of them. One of the last and firmest defenders of phlogiston was Priestley (1733-1804). Having invented suitable apparatus, similar to the pneumatic trough of the present day, he was enabled to collect and observe the properties of all the more important gases, the number of his discoveries in this field being truly remarkable.

Most important among them was the preparation of oxygen from the red oxide of mercury by means of heat, although Priestley himself foiled to appreciate the true value of his discovery. He, however, observed that this gas is given off by growing plants, whence he concluded that the latter thus replace the oxygen which has been removed from the air by combustion, etc, and that the two processes are in equilibrium with each other, the constant composition of air being thus maintained. Of equal importance for the development of the present system, though exerted in a different direction, were the labors of the Swedish chemists Bergman and Scheele. The method of analysis by the wet way introduced by Boyle had been but little followed till Bergman (1735-'84), carrying out the idea, established a complete series of reagents, and taught their use. He thus laid the foundation of the present system of inorganic analysis. He saw the advantage to be gained by causing each ingredient of a compound to unite with some other body with which it formed a combination of known constitution, capable of easy separation, rather than seek to isolate and determine it as such. Bergman analyzed a great number of substances, and investigated the composition of many salts.

But although possessing such correct views, he was an indifferent analyst, not equal even to some of his contemporaries. His reputation was so great, however, that many years elapsed before any correction of his results was allowed, though many of them have been since proved erroneous. He made numerous important discoveries; correctly explained the difference between cast iron, wrought iron, and steel, as well as the composition of many salts previously misunderstood. His most important work was upon chemical attraction (affinity). Some idea of its magnitude may be given by stating the fact that he drew up tables of the affinity of 50 different substances for each other, arranging them in two series according to their behavior when treated respectively in the dry or wet way. An enumeration of the original observations and discoveries of Scheele (1742-'86) would fill a volume. His investigations in organic chemistry alone are sufficient to prove him one of the best analysts that ever lived. He separated all the more common organic acids from plants, and knew how to distinguish them when mixed with each other, having devised processes for their separation. Several acids of the animal economy did not escape him.

He detected the presence of glycerine in the fats, separating it by means of oxide of lead, and prepared prussic acid from prussian blue. He also discovered molybdic and tungstic acids. Most fruitful was his research on the minerals of manganese, which, when the means at his disposal are considered, must be regarded as standing without parallel in the annals of chemistry. He discovered, first, the presence of anew metal (manganese); second, on adding muriatic acid to the black oxide of manganese, on which he was experimenting, a peculiar gas (chlorine) was evolved, the properties of which he accurately described. It is worthy of remark that he named it dephlo-gisticated muriatic acid, and at that time phlogiston was synonymous with hydrogen, showing that he regarded it in the same light as chemists do now. Thirdly, the ore of manganese on which he operated happened to contain a quantity of baryta, which he separated, and, having studied its properties, recognized as a new and peculiar substance. In solutions of its salts he found a test for sulphuric acid, which has since been universally used. Equally able was his investigation of fluor spar; he found it to be a compound of lime with a peculiar acid, which destroyed his vessels so rapidly that he was unable to collect it.

He investigated atmospheric air in its relations to combustion, finding that it contains two different gases, one of which, "empyreal air" (oxygen), is capable of supporting combustion and respiration, while the other, "vitiated air" (nitrogen), cannot maintain these processes. He proved that the metals, when burned to calxes, absorb empyreal air, while the calxes give it off when reduced to metals. He prepared oxygen at about the same time as Priestley, entirely independently, as admitted by Priestley himself, having obtained it from peroxide of manganese and from saltpetre as well as from the oxides of silver and mercury. The fact that the phlogiston theory had become inadequate to explain many points already known in chemistry, is more fully ex-hibited by Scheele than by any other chemist. He ostensibly believed in this theory, yet observation had taught him that oxygen is absorbed by metals during calcination and by combustibles when burning. To explain this, he conceived that oxygen is a compound of water with a certain light saline matter, in which compound but little phlogiston is contained.

During combustion the phlogiston of the combustible unites with the saline matter of the oxygen, producing light and heat, while the residual product is a compound of the matter which had originally been combined with phlogiston in the combustible, with the water of the oxygen. This theory, it will be seen, differs most essentially from that of Stahl, who saw in combustion nothing but a separation of phlogiston, while Scheele regarded it as a mutual decomposition of the combustible and the substance supporting combustion, new compounds being produced. - These views clearly show the transition state of chemistry at that period. In general, the ideas which the several chemists attached to phlogiston toward the end of the 18th century were far from exhibiting the accordance which had previously existed. The main feature of the phlogiston theory consisted in regarding combustion as depending upon decomposition, while chemists have since learned that it arises from the formation of compounds. It however explained the analogies and mutual relations of numberless phenomena which had previously been known as mere isolated facts. It was the most rational theory of combustion, and of many analogous processes now referred to oxidation and reduction, which had been proposed.

In a comparatively short time it led to a multitude of discoveries. By regarding bodies as composed of undecomposable elements, it established the view which has ever since been admitted. Indeed, the definition of chemistry given by Staid, the art of decomposing compound bodies and of reproducing them, is entirely in accordance with the spirit of the present day. Far from lamenting the phlogiston theory as an error, it should be regarded as a necessary basis for the views of the present epoch. In order to obtain a correct knowledge of substances, it was essential that their properties and relations to one another should be first investigated qualitatively. In the accomplishment of this work lies the merit of the phlogiston period. The quantitative method of investigation which followed, and which is characteristic of the present epoch, was but a natural succession. In no instance, however, in the history of chemistry has the transition from one system to another been so abrupt as from phlogiston to anti-phlogiston. Rarely has the introduction of an entirely new doctrine been so completely dependent upon one individual as was that of the present system upon Lavoisier (1743-94). He first caused the importance of the quantitative method of research to be recognized, having by its aid perceived that the increase of weight acquired by a given weight of metal, when oxidized, must disprove the whole theory of phlogiston.

The mere fact of this increase of weight had long been known; even Stahl was familiar with it. He regarded it, however, as entirely unessential and dependent on accidental circumstances. His followers were of the same opinion. One or two chemists had indeed called attention to the fact that this increase of weight was caused by an absorption of gas, but they had not themselves perceived the true importance of their observation, and their explanations had remained unnoticed. When at length the more careful investigation of Lavoisier had clearly proved the fact that the oxidation of metals is accompanied by increase of weight, the most absurd propositions were resorted to by the supporters of phlogiston. Among them it is only necessary to instance the hypothesis that phlogiston possessed absolute levity, being a substance endowed with a peculiar repulsive force, tending to remove it from the earth instead of gravitating toward it like other substances; whence a body losing phlogiston must become heavier.

Lavoisier not only exposed the error of supposing that the heavier metallic oxide could be an ingredient of a metal possessing less weight, but also framed a new theory in explanation of the facts, viz.: that in calcination, as in combustion generally, one of the ingredients of the atmosphere unites with the combustible in such proportion that the products of combustion weigh exactly as much as the sum of the weight of the substance consumed plus the weight of the matter absorbed from the air. Still more important was his proof that wherever an increase of weight occurs combination must have taken place; that the weight of the product of such combination is equal to the sum of the weights of its ingredients, while diminution of weight is invariably owing to separation of ponderable matter. As an investigator Lavoisier stands preeminent. His precision of observation, ingenuity in devising apparatus, and his patience, are only equalled by the clearness of his conclusions and his masterly description of facts. His original experiment, showing that the increase of weight which a metal acquires when calcined is caused by its combination with a gas, deserves to be mentioned. A known quantity of metallic tin having been placed in a retort, the latter was sealed up and the whole weighed.

Heat was then applied until the tin, having melted, was converted to a greater or less extent into a calx. On again observing the weight of the apparatus, it was found to have remained unchanged. But on opening the retort, air rushed in, and on again weighing an increase of weight was found to have occurred. This excess of weight showed how much air had entered the retort to replace what had been absorbed, and it was found that the weight of the tin had been increased by the same quantity. After the discovery of oxygen by Priestley, Lavoisier refined this experiment by using mercury in place of tin. Continuing his observations on oxygen, he found that carbonic acid is a compound of it and carbon, and explained the combustion of organic substances. He also studied the composition of sulphuric and other acids, seeking to multiply proof of his view that oxygen is the universal acidifying principle. Cavendish's discovery that hydrogen when burned forms water, afforded Lavoisier a clue for the explanation of the solution of metals in acids, which had previously been the weak point of his theory. He recognized at once that decomposition of water must take place, its hydrogen being set free, while the oxygen unites with the metal.

He also prepared water synthetically, and analyzed it by passing its vapor over red-hot iron, with which the oxygen united while hydrogen was set free, thus fixing its composition beyond the possibility of doubt. Lavoisier distinguished among the chemical elements those which ought to be regarded as simple, their further decomposition being improbable, as light, heat, oxygen, hydrogen, and nitrogen. Others he considered not so much simple as undecom-posed, their ingredients not yet being known. In this class he placed the alkalies, earths, and metals. The radicals of the acids he supposed to be simple; of these, sulphur, carbon, and phosphorus were known, while the radicals of boracic, muriatic, and hydrofluoric acids were merely hypothetical. He also sought to analyze organic substances by converting them into carbonic acid and water, thus originating the method now used, the means of executing which have alone been changed. The views of Lavoisier were greatly advanced through the cooperation of his contemporaries and countrymen, Guyton de Morveau (1737-1810), Fourcroy (1755-1809), and Berthollet (1748-1822). With Guyton de Morveau the idea of a rational nomenclature originated, and by his efforts the system now used was produced.

The naming of chemical substances had previously been governed by no rules whatsoever, the name given to a new compound depending entirely on the taste and humor of its discoverer. With few exceptions no name ottered any clue to the chemical properties of the substance. Guyton de Morveau first attempted in 1782 to express an idea of the composition of a substance by its name. This met with violent opposition from all sides. Lavoisier, however, had keenly felt the want of a systematic nomenclature; he therefore accepted the proposition to combine his new theory with the new nomenclature. Associating himself with Guyton de Morveau, Berthollet, and Fourcroy, a system was produced so nearly perfect that it only recently experienced a change in spite of the immense development which the science has since undergone, and the discovery of many substances, the existence of which could not have been foreseen at the time of its formation. Berthollet was the first chemist of importance who adopted the views of Lavoisier. He afforded them most important aid by many admirable experiments and acute investigations.

Although he manifested his own independence by refusing to admit that oxygen is the only acidifying principle, two acids, hydrosulphuric and hydrocyanic, which contain none, being already known, he was nevertheless led into error by Lavoisier's assumption that muriatic acid was composed of an unknown radical united with oxygen; conceiving that chlorine was composed of the same radical combined with more oxygen, while the acid which he discovered in chlorate of potash was supposed to contain this radical combined with a still greater quantity of oxygen. This error was not corrected until long afterward. Of the works of Berthollet, that upon affinity was of most importance for theoretical chemistry. While he admitted that all substances have really different degrees of affinity for each other, he sought to prove that what was commonly called affinity depended in great measure upon the relative quantity of the bodies acting on each other (force of mass); moreover, that the phenomena of decomposition attending such action depend essentially upon the physical properties of the compounds which are formed.

Since affinity can act only through direct contact of the most minute particles of matter, a body may be removed from the field of chemical action either by its insolubility (preponderating cohesion), or by its escape in the gaseous form (elasticity). According to him, there is no reason why two bodies cannot unite in all possible proportions to form chemical compounds, if their cohesion and elasticity, as well as those of the resulting compounds, are equal. The very thorough manner in which Berthollet explained all existing facts by means of this theory, combined with the strictness of his conclusions, caused it to receive at once universal attention. A few of his views have been indeed disproved, but as a whole it has since exerted a highly important influence on chemistry. The intimate dependence of the phenomena of decomposition upon the physical character of the resulting compounds, and the great influence which mass exerts in most reactions, have been universally recognized. Berthollet made several important researches. He determined the composition of ammonia, discovered fulminating mercury, and contributed much to the existing knowledge of prussic acid, chlorine, and hydrosulphuric acid.

The technical applications of chemistry which he brought about were numerous and exceedingly important; preeminent among them is the use of chlorine in bleaching. - The political condition of France toward the close of the 18th century exerted a most decided influence upon the progress and direction of chemistry, especially in its application to the arts, and its diffusion as a branch of popular knowledge. Previous to this time, technical chemistry was nothing but a collection of empirical facts, uncared for by scientific men, and all improvements in the arts depending upon chemical processes were the result of accident. But when France, a country accustomed to purchase from other nations her most important munitions of war, was cut off from outside communication, and compelled to defend herself against all Europe, scientific men, and especially chemists, were called upon to point out the means of producing the materials of war on which the very existence of the nation depended. They were asked to bring forth in a day arts which in other countries had resulted from the experience of years. There was no time now for a repetition of the groping empiricism by means of which these arts had been created. Science has rarely answered practical questions so quickly and clearly.

Not only were the requisite munitions soon prepared, but many arts were developed to an extent previously unknown. A knowledge of the sciences thus came to be considered of great importance for the welfare of the nation. As soon as law and order had been in a measure restored, new institutions for instruction were formed, to replace those which had been destroyed, in which the study of mathematics and of the physical sciences was made preeminent. This purely material direction has been thoroughly carried out in several of the most renowned schools of France. The influence which it has exerted in diffusing chemical knowledge is incalculable. Since the latter has become a matter of popular education, the methods of teaching it have been made subjects of special study and have been vastly improved. The immense influence which chemistry now everywhere exerts upon arts and manufactures, and which is one of the characteristics, not only of the science itself, but of the civilization of the 19th century, may be traced directly back to the labors of Berthollet, Guyton, and their associates. In connection with these, Fourcroy deserves mention.

He devised the plan of the system of instruction introduced into France by Napoleon, and did good service as a lecturer and writer. - Attention having been specially directed to quantitative analysis by Lavoisier and Bergman, many chemists now occupied themselves with it. Of these, Klap-roth (1743-1817) in Germany, Vauquelin (1703 -1829) in France, and Proust (died 1826) in Spain, exerted the most influence. Klaproth was the first German chemist who admitted the correctness of Lavoisier's views. He did much to diffuse them among his countrymen, in spite of the national feeling brought to bear against the "French system." But his chief merit is as an analyst, He first introduced the custom of publishing the results of quantitative analyses, as found directly by experiment. Any loss or excess had previously been distributed among the several ingredients, and such corrected values alone published, leaving no clue from which other chemists might judge of the accuracv of the statement. This led to numerous errors, many false notions of the composition of bodies having long been held on the authority of a single analyst.

The utility of Klaproth's system of publishing details has been most clearly proved by the fact that many of his own analyses have coincided with corrections which it has since been found necessary to apply to the inferences he had himself drawn from them. Klaproth devised the more common methods of decomposing insoluble minerals, which are still used. He pointed out the influence which the gradual destruction of the utensils in which analyses are made exerts upon the results obtained, and called attention to the necessity of applying a correction on account of it. Many of the methods now used for separating bodies from each other have come down from him. He discovered the oxides of uranium, zirconium, cerium, and titanium, and made many other important observations. Vanquelin performed a great amount of analytical labor, especially in regard to minerals. With Klaproth he had widely extended the field of analysis. But although they were agreed that most bodies have a constant or nearly constant composition, they were silent when Berthollet advanced an opinion that the reverse is often the case. Berthollet admitted but few compounds of constant composition in one proportion. In most bodies he thought the constituents capable of uniting in any proportion between two limits.

Thus, iron could unite with oxygen in any proportion between protoxide and peroxide. This view did much mischief. Every false analysis supported itself, while it seemingly supported the theory, upon it, by admitting that the combining proportions of the ingredients were variable. Proust deservedly won great reputation by proving that these supposed intermediate compounds do not exist in so many varying proportions. He demonstrated that when two substances unite in several proportions, the compounds formed are but few and are separated from each other by intervals, never gradually shading into each other. lie explained correctly the composition of red lead, of magnetic oxide of iron, etc. He pointed out the errors committed by previous investigators of the subject, and the necessity of not confounding chemical compounds with mechanical mixtures. His views were soon received as correct by chemists, in spite of the opposition of Berthollet. He also carefully studied several metals and hydrates of metallic oxides, distinguished grape from cane sugar, etc.

Proust and his predecessors, in determining the composition of bodies, sought only to ascertain how much of each ingredient was contained in a constant weight, usually 100 parts of the compound, thus referring the weight of the ingredient to that of the compound. The science had thus been greatly advanced, but still more important discoveries were made when chemists began to consider the relations which the weights of the several ingredients of a body bear to each other, and to investigate how much of one substance is required to replace another in a compound. The idea of chemical equivalents thus arose, and it was soon recognized that chemical combinations take place not only in constant but also in simple relations of weight. Wenzel in 1777, and Richter in 1792, in Germany, were the first who endeavored to call attention to this subject. The former explained the fact that when two neutral salts mutually decompose each other, the resulting mixture is still neutral, by admitting that a quantity of either base sufficient to neutralize one acid could also neutralize the other acid. He moreover showed that the relative weight of two bases which neutralize the same quantity of acid remains constant, no matter what acid may be used.

Arguing from Wenzel's law, Richter showed that, in accordance with it, the composition of all the neutral salts of one acid and of any one salt of any other acid having been ascertained by analysis, the composition of any other salt of this acid could be calculated, He also showed that numbers could be affixed to the acids and bases which would express the relation of weight in which they combine with each other to form neutral salts, and even constructed such tables of equivalents. The views of both these chemists were neglected until the publication of the atomic theory of Dalton (1766-1844) recalled the attention of chemists to them, when they exerted no inconsiderable influence in establishing Dalton's doctrine. This last was much more extended than the views of either of the chemists just mentioned. Having observed that when a de-. termined quantity of any substance united with different quantities of a second substance, the quantities of the latter always bore a simple relation of weight to each other, Dalton was led to the formation of his atomic theory. He regarded the elements as composed of homogeneous atoms, the weights of which are different for each different element.

An atom of one element can unite with one or more atoms of another element, the weight of the atom of the compound formed being the sum of the weights of its component atoms. He determined the relative weights of these atoms for the different elements, as expressed by the relative weights in which they unite to form compounds. Although the term atom used by Dalton expresses nothing more in a chemical sense than Rich-ters word equivalent, there was a tangibility in the former which caused the view to be more readily accepted. Moreover, Dalton's discovery of the law of multiple proportions, and that the atomic weight of compounds is the sum of the atomic weights of their ingredients, made the subject so complete that it could no longer be neglected. It was at once admitted into the science, and gave rise to the views concerning the quantitative composition of bodies which now exist. Besides the atomic theory, chemistry owes much to Dalton for his investigations of the expansion of gases, evaporation, and the relations of mixed gases, elasticity of steam, etc. AVollaston (1766-1828) did much to diffuse and extend Dalton's theory. His "scale of chemical equivalents" especially aided in this result. As an exact analyst also, he was deservedly celebrated.

Most important in connection with Dalton's doctrine was the discovery by Gay-Lussac (1778-1850) of the law of combining volumes, in accordance with which gases unite with each other. In conjunction with Humboldt he first observed that one volume of oxygen unites with two volumes of hydrogen to form water. Extending his researches, he found that other gases unite in equally simple proportions, and that the volume of the resulting compound, if gaseous, bears a simple relation to the sum of the volumes of its ingredients. This proved conclusively that chemical compounds are formed only in a few fixed and definite proportions, according as it did with Proust's researches upon the composition of solid bodies. It was soon perceived that, the specific gravity of a gas being known, its atomic weight might be readily calculated, whence the determination of the density of gases became at once important. Of the many valuable investigations conducted by Gay-Lussac, those upon the expansion of gases by heat, density of vapors, for the determination of which he devised an apparatus, expansion of liquids, evaporation, sulphur acids, chlorine compounds, and iodine may be mentioned. His researches in organic chemistry were also very important.

He first pursued the method of research which has since exerted so great an influence upon the development of chemistry. The investigation of the cyanogen compounds and isolation of cyanogen, a compound of nitrogen and carbon, yet closely resembling in its properties the element chlorine, gave rise to the idea of organic radicals, and formed the starting point to which the present method of regarding organic bodies directly refers. He first devised an apparatus for the ultimate analysis of organic substances which produced useful results, infinitely superior to any previously obtained, and furnished a basis for improvement. He also introduced the system of determining the specific gravity of the vapors of substances to control analyses. Many of his applications of chemistry to the arts were of great importance; among them are the methods of assaying silver by the wet way, of alkalimetry, of chlorimetry, of assaying gunpowder, etc, still in use. - Simultaneously with the investigations upon the atomic weights and atomic volumes, which occupied chemists alter the discoveries of Dalton and Gay-Lussac, another subject of great importance in its bearings upon the doctrine of affinity, and which produced most striking results when applied to the study of certain individual substances, was brought forward.

This was the investigation of the connection between galvanism and the phenomena of affinity. When Berzelius and Hisinger began to study this subject in 1803, it had already been noticed that water could be decomposed by the galvanic current. They verified this experiment, and moreover showed that salts could be decomposed by the same means; while an electrical opposition between acids and bases was indicated by the fact that under all circumstances of decomposition acids were set free at the positive pole, bases at the negative pole, of the voltaic pile. The subject rested somewhat obscure, however, until cleared up by Sir Humphry Davy (1778-1829), most of whose labors were devoted to electro-chemistry. He first showed by a most admirable research that pure water when decomposed by galvanism produces only hydrogen and oxygen; that the acids (nitric and muriatic) and bases (ammonia and soda) obtained by previous experimenters had been produced either from air contained in the water or from the action of the galvanic current upon the vessels used in the operation. Studying this last action more carefully, he eventually succeeded in separating metals from the fixed alkalies, potash and soda, and proved these last to be metallic oxides.

Small as were the quantities of metal thus obtained, he described their properties with surprising accuracy. He afterward proved that the alkaline earths are similarly constituted, and inferred from analogy that the earths are of the same nature; the correctness of which opinion has since been fully proved. Having studied the chlorine compounds, he was led to regard chlorine as an element, and disproved Berthol-let's view of its composition, which had hithorto been universally received. This gave rise to the first modification of any importance which Lavoisier's system Buffered. Oxygen could no longer be regarded as the sole acidifying principle. The idea of hydrogen acids was introduced, and substances which contain no oxygen admitted to be salts. Of the many other researches conducted by Davy, that upon flame and combustion, which led to the discovery of the safety lamp, was especially valuable. His electrical theory has been since modified, but, like most of his work, has been of immense importance to the science. The value of his discovery of the metals of the alkalies was not limited to a theoretical bearing alone. From the enormous affinity for oxygen which they possess, a means was furnished far stronger than any previously known of decomposing other bodies.

They did not obtain their full importance as reagents, however, until a process of manufacture was devised by Gay-Lussac and Thenard, by which they were obtained in comparatively large quantities. By them the chemists last mentioned were led at once to the discovery of boron, of hydrofluoric acid, of fluoride of boron, etc. Since then they have been the means of many discoveries. As an investigator and diffuser of chemical knowledge, as well as by his systematic classifications, Thenard (1777-1857) was of great service to the science. His division of the metals into groups, according to their behavior at different temperatures in presence of water, has been quite generally followed. Among his important discoveries may be mentioned that of the peroxide and of the persulphide of hydrogen. - No one chemist since Lavoisier has exerted an influence comparable with that of Berzelius (1779-1848). In him were united all the different impulses which have advanced the science since the beginning of the present epoch. The fruit of his labors is scattered throughout the entire domain of the science. Hardly a substance exists to the knowledge of which he has not in some way contributed. A direct descendant of the school of his countrv-man Bergman, he was especially renowned as an analyst.

No chemist has determined by direct experiment the composition of a greater number of substances. No one has exerted a greater influence in extending the field of analytical chemistry. The use of hydrofluoric acid in decomposing minerals, and of chlorine in the analysis of minerals containing metals capable of forming volatile chlorides, originated with him. Of his manifold and always admirable researches may be mentioned those upon the organic acids; upon selenium, which he discovered; upon the alkaline sulphides, in which the theory of sulphur salts, analogous to those of oxygen, was brought forward; upon the fluorine compounds; upon platinum and the metals occurring with it; upon tellurium; upon meteorites, and upon the silicates. He first isolated several substances, as silicon, zircon, tantalum, etc. The remarkable amalgam which mercury forms with what is supposed to be ammonium was first obtained by him in conjunction with Hisinger. One of the principal services he rendered was in developing the present theory of the science.

Before Dal-ton's views had become generally known, Berzelius had perceived the importance of Richter's tables of the combining equivalents of acids and alkalies, and had carried out an extended investigation of the composition of salts to ascertain if they were true. The results obtained convinced him at once of the correctness of Dalton's generalization, which soon afterward came to his knowledge. He continued his researches, and worked out most fully the details of the subject. Among other things, he thus discovered the simple relation which the oxygen of the acid bears to the oxygen of the base in neutral salts. He also endeavored to ascertain with the greatest possible exactitude the relations of weight in which the different elements unite to form compounds. His acuteness in selecting the materials best suited for his experiments, and the precision with which their quantitative analysis was conducted, have never been surpassed. He did not limit himself to prove these equivalent weights for only a few substances. By far the greater number of the elements were investigated by him, and a large proportion of the equivalent numbers still in use are his.

Corrections have been made only by those experimenters who have been able to procure purer materials; as an accurate analyst, Berzelius has never been surpassed. He first demonstrated that organic bodies combine according to the equivalent weights of their ingredients, and introduced the method of analyzing their compounds with inorganic bodies of known equivalent weight as a means of arriving at their equivalent. He maintained most per-sistently the view that organic substances form combinations analogous to those of inorganic bodies, and that they ought to be explained in accordance with what is known of the latter, having steadfastly opposed the innovations on this doctrine brought forward by other chemists. He made, however, numerous accurate analyses of organic substances, and chemistry owes to him the theories of copulate acids and of organic radicals. The latter, in accordance with which a substance may be composed of two or more elements, and yet be capable of entering into combination with elementary bodies as if it were itself elementary - in a word, play the part of an element - has since exerted a most important influence upon the development of the science.

The admirable system of chemical symbols now in use also orginated with Berzelius. - Following in the footsteps of Davy, Faraday (1791-1867) was most fortunate in developing the relations of electricity to chemistry. He widely extended the idea suggested by Davy that electricitv and chemical affinity are only different expressions of one and the same force. All his experiments tended to support this view. Although most of his researches fall more properly within the province of physics, they are nevertheless of the greatest interest in their chemical hearings. Preeminent in this respect are his works upon the liquefaction of gases, and upon certain compounds of hydrogen and carbon, which last was one of the starting points of the doctrine of isomerism, also upon the compounds of carbon and chlorine, and of ammonia and metallic chlorides. Working like Faraday in the domain of both physics and chemistry, Mitscher-lich of Berlin (1794-1863) exerted a great influence on the present condition of the science by his discovery of the law of isomorphism, in accordance with which certain groups of substances exist, any one member of which can be replaced by any other member or set of members in equivalent proportion in its compounds, without changing the crystalline form of the latter to any material extent; and of dimorphism, the power possessed by some substances of crystallizing in two distinct systems not reducible to the same primary form.

Following as did these discoveries upon those of Faraday, and that of the French physicists, Dulong and Petit, of the relation between the specific heats and equivalent weights of substances, it tended greatly to call the attention of chemists to the physical relations of bodies. A physico-chemical school has thus been founded, to which several of the leading chemists of the present day belong. The discoveries of Mitscherlieh, the details of which he worked out by a series of most laborious researches, were soon brought to bear with advantage upon the classification of the elements, while that of minerals underwent an entire reform. - The theory of compound radicals proposed by Ber-zelius was again made specially prominent by the publication (in 1832) of a memoir, the joint production of Liebig (1803-'73) and Wohler (born 1800), upon the benzoyl series. Although Berzclius refused to admit the generalization made by these chemists, and disbelieved in the existence of radicals which like benzoyl contain oxygen, an interest was nevertheless excited, which greatly contributed to the advancement of organic chemistry. The ethyl theory quickly followed, and was adopted by most German and English chemists. These results mark an era in the history of modern chemistry.

A throng of pupils immediately gathered about Liebig. No other chemist has ever had control over an amount of talent equal to that of the students who for years crowded his laboratory at Giessen. Through them he exerted an incalculable influence upon the present position of the science, while by his popular writings he did much to diffuse chemical knowledge among the masses. Of the special labors of Liebig may be mentioned his efforts to determine what substances should be regarded as radicals, and to classify all known organic bodies in accordance with them; also his important improvements in the methods of analyzing organic substances. Besides the investigation of the oil of bitter almonds, which led to the discovery of the existence of benzoyl, his most important researches were made in conjunction with Wohler; for example, those upon cyanogen compounds and the derivatives of uric acid, by which an immense number of new compounds were discovered. Wohler has also independently brought to light many new facts, not only in organic but also in inorganic chemistry; for example, his investigation of the compounds of tungsten, the preparation of aluminum and of glucinum, and, in conjunction with H. Deville, of silicon and boron.

His method of preparing urea (1828) from inorganic substances was the first and for years the only example of the power of chemists to form organic compounds from their elements. All organic substances previously obtained had been either derived directly from plants or animals, or bad been products of the decomposition of substances thus obtained; it had indeed been doubted if any others could ever be prepared. - The labors of the recent French chemists have aided perhaps more than any others in elevating the science to its present position. Although seemingly standing for years in direct opposition to those of the German school, the systems of both have at length been harmoniously combined. Dumas of Paris first discovered, by his research upon the action of chlorine on acetic acid, that the three equivalents of hydrogen contained in the latter can be replaced by as many equivalents of chlorine, while the acetic acid retains most of its characteristic properties. Upon this and similar observations he founded his theory of substitutions, according to which hydrogen and some other ingredients of compounds may be replaced, equivalent for equivalent, by some other element or group of elements, while the properties of the original substance are usually not essentially changed.

Few theories have been more bitterly opposed than this, but the facts are now universally admitted. The investigation of the substitutions which can be produced in organic compounds has been for several years a favorite study with chemists. Of late the means of bringing them about, and of reproducing the primary substance by replacing the elements originally removed, have been greatly increased and have led to many fine discoveries. Numberless researches upon organic substances have been conducted by Dumas with very important results. His determinations of the specific gravity of many vapors were of great value, the ingenious apparatus which he devised for this purpose having since been almost universally used. Determinations of the specific gravity of gases made in conjunction with Boussingault were also valuable.' Among his most important investigations are those upon the amidogen compounds, ethers, volatile oils, and especially upon wood spirit, made in conjunction with Peligot, by which its resemblance to alcohol was shown - an analogy carried yet further by Balard's discovery of amyl alcohol, which suggested the idea that the term alcohol must be regarded not merely in a specific but in a generic sense.

Similar analogies were soon observed among other organic compounds, and it gradually became evident that very many organic substances could be classified together in homologous series, the composition of each member of which differs from that of the others, either by a certain number of equivalents of carbon and hydrogen, or by some simple multiple of this number; while the chemical properties of the several'members of the scries are entirely analogous, differing only in degree in direct proportion to the amount of carbon and hydrogen which they contain; whence, the general properties and relations of any one member of a series being known, those of any other of its members may be directly inferred. Remarkable relations between the physical properties of the different members of the same homologous series have also been noticed. Thus, II. Kopp has observed that the point at which the several members of a series boil increases about 19° centigrade for every two equivalents of carbon (old value) plus two of hydrogen which they contain more than the first member of the series.

It is admitted by Kopp that the double atom of carbon elevates the boiling point 29° 0., while the double atom of hydrogen diminishes it 10° 0. The fusing point also of the members of several series presents analogous relations, while Kopp has shown by a number of most important investigations upon atomic or specific volumes (by which terms are understood the relative spaces occupied by the atoms or equivalents of bodies, being the quotients obtained by dividing their equivalent weights by their specific gravities), that the atomic volumes of many homologous compounds differ from each other by a constant quantity, proportional to the number of double atoms of carbon and hydrogen which they contain. The credit of the first clear perception of this wonderful system of homol-ogism is due to Gerhardt (1816-56). No system has done more to advance organic chemistry than this, while no one of his contemporaries has surpassed its gifted and laborious author. Of the chemists who especially devoted themselves to the advancement of the doctrine of substitution, no one can be compared with Laurent (1807-53). Denouncing compound radicals as purely hypothetical bodies, he endeavored to substitute for them his own theoretical nuclei. In itself this theory has exerted comparatively little influence.

In the hands of Laurent, however, it led to the discovery of an immense number of new compounds, the very naming of which required that a special nomenclature should be framed. Few works of recent times have displayed greater originality than his Methode de chimie. The comparative harmony now existing among chemists in their views regarding organic substances was in great measure brought about by the investigations of the compound ammonias and ammoniums by Wurtz of Paris and Hofmann of London. Kolbe of Marburg and Frankland of Manchester had indeed isolated bodies of the same composition as methyl, ethyl, and other radicals, but their properties were less active than chemists had expected, and their identity with the true radicals was not at once admitted. The question was, however, settled by Hofmann's discovery that one or all of the equivalents of hydrogen in ammonia may be replaced by an equal number of equivalents of one radical possessing basic properties, like ethyl or methyl, or by several such radicals, so that four different radicals may be present in one equivalent of ammonium. Chemists were now forced to admit that the radicals entered into combination precisely as if they were elements, replacing hydrogen equivalent for equivalent.

In like manner Gerhardt demonstrated that one or more of the three equivalents of hydrogen in ammonia may be replaced by a compound radical containing oxygen, as benzoyl; thus proving the correctness of the original benzoyl theory, which had suffered so many attacks. The extension given by these researches to the idea attaching to the term ammonia was immense. It became at once generic, including innumerable bodies, all possessing to a greater or less extent the characteristic properties of the original ammonia. Among the most interesting types of organic compounds are those in which two bodies of analogous nature are united together in such a manner that the properties of the compound resemble in kind those of one of its ingredients, the original properties of the latter being modified only in degree. Such compounds are called copulate or conjugate, and may be either acids, bases, or radicals. Many of the copulate acids were studied by Berzelius and his immediate followers, while the conjugate radicals have been especially investigated by Frank-land, Lowig of Breslau, and others, among whom Prof. Wolcott Gibbs of New York and Dr. F. A. Genth of Philadelphia deserve special mention The first investigation of this subject, and one of the finest in the records of chemical science, was that by Bunsen of Heidelberg on kakodyl, a radical composed of two equivalents of methyl united with one of arsenic.

Of deep interest is the success which has recently attended attempts to prepare organic compounds by combining others of more simple composition. For a long time urea was the only compound which could be thus prepared, but within the last few years many such have been obtained. In this department, the experiments of Berthelot of Paris are most important. By heating carbonic oxide gas with hydrate of potash in a sealed vessel, he has obtained formiate of potash, from which formic acid may be readily prepared in a free state. By agitating olefiant gas (one of the components of ordinary coal gas) with oil of vitriol, a compound is formed from which, on addition of water and distillation, alcohol is obtained. By analogous processes, any of the homologues of olefiant gas being substituted for it, the corresponding alcohols may be prepared. More than half a century ago Chevreul by a classical research proved that the fats are composed of various acids united with a peculiar sweet base, called glycerine. Subsequently Berthelot succeeded not only in recombining the fat acids with glycerine, thus forming the original fats, but also caused all the more common mineral and organic acids to unite with glycerine in a manner precisely analogous.

Moreover, by substituting for glycerine any of the various species of sugar, similar combinations have been obtained with the fatty and other acids. The indefinite extension which has thus been given to the chemical idea of the fats is a good example of the spirit of generalization now greatly in vogue. - The comparatively immense development which the study of organic compounds has taken of late years has at length brought chemical science to such a position that a new epoch seems not remote. The laws by which the chemical relations of inorganic compounds have hitherto been so well explained, fail in many cases when applied to organic substances. The domain of organic chemistry has become so vast that it will soon assert its right to control the whole science. Several chemists have called attention to the apparent necessity of such reformation. It is acknowledged on all sides that the artificial division of chemistry into organic and inorganic, which has been admitted for some years, is entirely arbitrary, and in many respects unfortunate. That the science shall once more be consolidated is earnestly desired.

The doctrine of substitution has already cleared up many matters in inorganic chemistry which had hitherto been inexplicable; while the question of doubling the equivalent weights of several of the elements in order to bring them into accordance with their combining volumes and certain other of their properties is still open. The extraordinary analogy between homologous groups of organic compounds and certain small groups of the elements, as chlorine, bromine, and iodine, has been remarked by several chemists. It has been generalized by Prof. J. P. Cooke of Cambridge, Mass., who has shown that not only isolated triads, but all the elements, may be brought into such homologous series, expressed like those of organic compounds by the general formula a + nb. The properties of the members of these series vary in degree in a regular manner according to their position, as in the series of organic bodies. Dumas criticised and in a measure modified Cooke's classification, while acknowledging its merit.

The bearing of this subject upon the relations of compound radicals to the so-called elements cannot fail to strike every observer; the difference between them consisting simply in the ability of chemists to separate the former into several ingredients, while the latter are elementary only because they cannot as yet be thus decomposed. - Without occupying themselves with the investigation of a problem, the transmutation of metals, to the solution of which their science in its present condition offers no clue, chemists have ceased to ridicule the aspirations of the alchemists, although they will always condemn the venal spirit which actuated them. The possibility of effecting such transmutation has of late, however, been more strongly suggested by the discovery of several remarkable examples of allo-tropism, a term employed to signify that the same body may exist under two or more different conditions, possessing distinct physical and chemical properties. The fact that bodies so entirely unlike in their properties as the diamond, graphite, and charcoal are, chemically speaking, identical, standing as it did for a long time an almost isolated example, excited comparatively little attention.

Nor was much notice taken of the different states of sulphur, and of other bodies the allotro-pism of which was less clearly apparent. But a deep interest was awakened by the discovery of ozone (allotropic oxygen) by Schonbein of Basel, and especially of red phosphorus by Schrotter of Vienna, bodies as unlike ordinary oxygen and phosphorus as can be conceived in every respect but their combining equivalents and reconvertibility into each other. The observation that the elements boron and silicon are, like carbon, susceptible of three modifications, strengthened this feeling. These discoveries recalled attention to the fact that several of the metallic elements have identical equivalent weights, as cobalt and nickel, platinum and iridium, etc.; the question naturally arising whether they may not be allotropic modifications of one and the same substance, especially as such modifications of several substances preserve their peculiarities even when combined in similar quantities with other bodies; for example, the compounds of the red and green varieties of the sesquioxide of chromium. Even the idea attached to the term element has been somewhat modified.

At one time it was regarded as expressing not only a certain relative weight of a simple substance, but this substance was supposed to possess constant properties, which were as indestructible as the element itself. Instead of this view, which is at present untenable, the old idea of essence has been in a measure restored. - It would be of course impossible to enumerate here all the valuable labors which have recently been performed by chemists. Those of II. Rose (1795-1864) in developing inorganic analysis; of Regnault, Bunsen, Kopp, and Magnus (1802-'70) in investigating the physical laws connected with chemistry, and of the first two of these chemists in perfecting the processes of gas analysis; of Rammelsberg, Pasteur, Pelouze, Redtenba-clier, Malaguti, Williamson, Heintz, Rochleder, Stadeler, Strecker, Cahours, Anderson, Kolbe, Draper, Wurtz, Kekule, Fresenius, and a host of others, in addition to those who have previously been alluded to, in the field of organic chemistry, deserve special mention.

The discovery of the anhydrous organic acids by Gerhardt may here be notieed, as well as H. Deville's process for preparing aluminum and sodium on the large scale, which promises to exert an influence in advancing chemical knowledge equal to that which resulted from the manufacture of potassium and sodium by Gay-Lussac and Thenard. - There is in modern times a general tendency to use the volumetric method of analysis, to employ the blowpipe for quantitative as well as qualitative analysis, and to apply the spectroscope to the detection of rare substances. The nomenclature of chemistry has undergone a remarkable change during the past few years. Old and familiar names have been dropped, and others more in accordance with the modern notions of the true composition of bodies have been substituted. The doctrines at first timidly advanced by a few chemists have gradually been accepted by a majority of scientific writers, and the whole language of the subject is now in a transition state. The equivalent value or combining capacity of an element is now measured by the number of atoms of hydrogen or other monatomic or univalent element with which the element in question can combine. Chlorine, which unites with one atom of hydrogen, is monatomic, monadic, or univalent.

Oxygen, which combines with two atoms of hydrogen, is diatomic, dyadic, or bivalent. Nitrogen, which combines with three atoms of hydrogen, is triatomic, triadic, or trivalent. Carbon, which combines with four atoms of hydrogen, is tetratomic, tetradic, or quadrivalent. The elements are divided into two classes, one of odd, the other of even equivalence, the former distinguished as peris-sads, the latter as artiads; e. g.:

Perissads.N, P, As, Sb, Au.

Artiads..O, S, Se, Te, Ba, Ca, Mg, Sn, Mo, W, etc.

The nomenclature of compounds has been adapted to the new order of things, and instead of saying carbonic acid, the compound is now called carbon dioxide or carbonic anhydride. In a work of this character it is preferable to retain the names of substances by which they have long been known, but the modern appellations will always be given. - The details of chemical science are treated in the articles Affinity, Atomic Theory, ELEMENT, EQUIVALENT, ISOMERISM, NOMENCLATURE, Symbol, etc, and under the titles of the different animal, vegetable, and mineral substances.