Diathermancy (Gr. sta, through, and OEpua, heat), permeability to the rays of heat. Dia-thermanous bodies have the same relation to calorific rays that transparent ones have to rays of light; and those bodies which are impermeable to rays of heat, or bear the same relation to it that opaque ones do to light, are called athermanous. It was long known that rays of heat from an intense source were capable of passing through certain transparent substances, like glass, in lines subject to the same laws of refraction as those of light; but it was not supposed that such transmission was possible through bodies which are opaque to light. Pictet of Geneva was the first to show that radiant heat, from obscure as well as from luminous sources, would pass through plates of different transparent substances. But it was even then believed by many that the heat was communicated by being absorbed and subsequently radiated by the transmitting body, until Prevost of Geneva proved the fallacy of this idea by passing rays of heat through ice, of sufficient power to ignite combustible substances.

The investigations of Melloni, however, placed the subject in a clearer light, and to him we owe most of the facts that other distinguished investigators (among whom are Bunsen, Kirchhoff, Tyndall, and Balfour Stewart) have elucidated by numerous brilliant experiments, which have given the. subjects of radiant heat and light so much interest at the present time. The apparatus used by Melloni in his experiments on the diathermancy of various bodies is represented in the engraving. Nobili's thermo-electric pile, made of alternate layers of bismuth and antimony, represented at a, connected by copper wires with a delicate galvanometer b, constituted the thermometer employed by Melloni to measure the radiant heat transmitted through the substances experimented upon, and which he termed his thermo-multiplier. The transmitted rays were received upon one of the faces of the pile, and generated galvanic electricity in proportion to their quantity, which was indicated by the galvanometer. The body whose diathermancy was the subject of experiment was placed upon an adjustable stand at c, while two screens, d and e, one for excluding the rays of heat until the test should commence, and the other for limiting the pencil of rays, were placed between c and the source of heat, f, which might be a Lo-catelli lamp, a coil of incandescent platinum wire, a heated metallic ball, a can of boiling water, or any other body heated to the desired degree.

During his investigations he made an important discovery, which has since been used to great practical advantage by Tyndall in many brilliant experiments upon the transmission of heat through gases and vapors, viz.: that rock salt is almost perfectly diatherma-nous to radiant heat from all sources, whether luminous or obscure. Indeed, he supposed it to be perfectly diathermanous, attributing the nearly constant loss of 7.7 per cent. of the heat falling upon it to reflection from the surface. In testing the diathermancy of liquids, Melloni placed them in narrow troughs of thin glass, and measured their transmitting capacity by the difference in the amount of heat which passed through them when empty and when filled with the liquid. The following table shows the percentage of heat transmitted by several of the substances with which Melloni experimented, using four different sources of heat. The experiments were made by ascertaining the deflection of the needle of the thermo-multiplier when the rays from each source were passed through air, and, calling this amount 100, passing the rays from each source through the various substances, and noticing the deflection produced.

Melloni's Apparatus.

Melloni's Apparatus.

Diathermancy Of Solids

SUBSTANCES, 1/10 of an inch thick.

Locatelli lamp.

Incandescent platinum.

Copper at 752° F.

Copper at 212° F.

Rock Salt.......

923

923

92.3

92 3

Siellian sulphur.......

74

77

60

54

Fluor spar.............

72

69

42

33

Beryl...........

54

23

13

0

Iceland spar...........

39

28

6

0

Glass..........

39

24

6

0

Rock crystal, clear...........

33

28

6

3

Smoky quartz..........

37

23

6

3

Chromate of potash.........

34

28

15

0

Carbonate of Lead.........

32

23

4

0

Sulpahate of baryta........

24

18

3

0

Feldspar...........

23

19

6

0

Borate of Soda.........

18

12

8

0

Selenlte................

14

5

0

0

Tartrate of potash.......

11

3

0

0

Alum..........

9

2

0

0

Sugar candy........

8

1

0

0

Ice........

6

05

0

0

These results show that transparency to rays of light and permeability to those of heat, or diathermancy, although they accompany each other to a certain extent, do not do so proportionately. Rock salt, it is seen, is equally diathermanous to all the sources named in the table. It is found, however, to be not perfectly diathermanous to rays of extremely low re-frangibility, as will be noticed further on. The difference exhibited by glass in transmitting heat from the different sources is very marked. Thus, while transmitting 39 per cent. of the rays from a Locatelli lamp, and 28 from incandescent platinum, it permits the passage of only 6 per cent. of those which are emitted from copper heated to 752° F., and is completely opaque to those issuing from copper at 212°. Again, clear rock crystal transmitted only 1 per cent. more heat from the Locatelli lamp than did smoky quartz, and no more from the other sources; a fact which shows how little heat is contained in the highly luminous rays of the spectrum. The low diathermancy of ice is here also shown; a property which adapts it, as well as the other forms of water, which share it in a like degree, to the various relations it sustains with organized life.

The following table, also from Melloni, shows the amount of transmission by various liquids, the source of heat being an argand lamp with a glass chimney, and the liquids 1/36 of an inch in thickness, held in glass cells:

Diathermancy Of Liquids

LIQUIDS, 1/30 of an inch thick.

Percentage of transmission.

Bisulphide of carbon............................

63

Bichloride of sulphur...........................

63

Protochloride of phosphorus.....................

62

Oil of turpentine................................

31

Olive Oil..........

30

Naphtha..........

28

Oil of lavender.............

26

Sulphuric ether.......

21

Alchol.......

15

Acetic acid..........

12

Concentrated solution of sugar........

12

Distilled water...........

11

A remarkable fact in relation to the diathermancy of bodies is that rays of heat which have once been transmitted by a substance will readily pass through a second plate of the same material with little or no loss; that is, glass is nearly diathermanous to heat which has already passed through glass, and ice is nearly diathermanous to heat which has passed through water or ice, or a considerable depth of watery vapor. Another very important fact, intimately connected with the subject of molecular physics, is that all bodies, solids, liquids, and gases, are nearly athermanous to heat which is radiated by the same body. Thus, rock salt, which is nearly diathermanous to all sources of heat, absorbs most of the rays that are radiated by heated rock salt. Balfour Stewart found that a moderately thick plate of cold rock salt would stop three fourths of the heat radiated from a plate of rock salt. This fact is accounted for on the wave theory, by supposing that the rays of very low re-frangibility, which are the ones radiated by this substance, have the power of exciting vibrations of the same wave length in the same material, and are therefore accepted or absorbed; whereas the rays of higher refrangi-bility, and consequently of shorter wave length, which most other bodies emit, are allowed to pass through rock salt because they have not the power, by reason of non-accordance, to set its particles into vibration.

From the fact that this substance only radiates heat of low refrangibility, it would be concluded that when heated it would require a long time to cool, and also that it would accept radiant heat slowly, although it is readily warmed by conduction; and this conclusion is borne out by experiment. The absorbing and radiating powers of bodies are reciprocal and equal, as has been shown by the experiments of Sir John Leslie, Ritchie, and others. The diathermancy of a body may therefore be stated as inversely proportional to its power of radiation. Athermanous bodies, or those which are only slightly diathermanous, are more permeable to rays of high than to those of low refrangibility; consequently, if the luminous-ness of a flame is increased, although it may contain no more heat, it will radiate more through partially diathermanous media, as for instance moist air, glass, and alum. Again, if its luminousness is decreased, these media will be more opaque to its rays; and if heat of still lower refrangibility is substituted for the flame, their opacity will be the more increased.

The investigations of Tyndall on the heat-absorbing powers of various liquids and gases, or in other words their relative diathermancy, have also thrown much light on the subject of the molecular constitution of matter. He has shown that elementary bodies are generally much more diathermanous than compounds. This has been used as a remarkable evidence in favor of the wave theory of light, because by adopting it the phenomena of transmission and absorption are perfectly accounted for, and in no other way. Placing a solution of iodine in bisulphide of carbon in a rock salt prism, he found that it transmitted 99 per cent. of all the rays emitted by a body heated below luminousness. Converging the rays which were transmitted through the solution, he found them as effectual in producing combustion as if the transmission had not been made. Iodine is therefore diathermanous to rays of obscure heat. The elementary gases and their mechanical mixtures he found to be almost perfectly diathermanous, while compound gases and vapors are partially so, many of them transmitting only rays of high refrangibility, or those belonging to the luminous spectrum.

The diathermancy of dry atmospheric air was found to be more than 250 times that of nitrous oxide gas, a chemical compound of the constituents of the air in the same proportion; and this he regards as one of the strongest proofs that the atmosphere is a mechanical mixture, and not a chemical compound. In experimenting upon the conductivity of different substances, Tyndall found that this property in a body was generally commensurate with its diathermancy, with one exception, which was that slightly diathermanous rock crystal was a better conductor than almost perfectly diathermanous rock salt. The latter substance has, however, a high conducting power; and it was found that rock salt, glass, calcareous spar, selenite, and alum maintained the same order of conductivity that they did of diathermancy in the experiments of Mel-loni. Some of the experiments made by Tyndall will be more particularly described in the article on Heat. The object he had in view made it necessary to employ apparatus which would allow of the transmission of the rays of lowest refrangibility, because it is these that are especially interfered with by vapors and compound gases.

His sources of heat were often metal surfaces heated with boiling water, or to a temperature far below redness, and the rays were passed through a tube whose ends were closed with plates of transparent rock salt. This tube could be exhausted before the gas or vapor was admitted, and the latter could be introduced through apparatus which excluded all moisture; so that many errors which have often affected the value of previous experiments were avoided. The following table shows the relative absorption of radiant heat of several different elementary and compound gases, and the vast difference in the degree of diathermancy which they possess; the transmission was made through each of the gases at the common pressure of the atmosphere:

Diathermancy Of Gases

GASES.

Relative absorplian.

Air............................................

1

Oxygen........................................

1

Nitrogen.............

1

Hydrogen.........

1

Chlorine........................................

39

Hydrochloric acid.......

62

Carbonic oxide.................................

90

Carbonic acid...................................

90

Nitrous oxide..................................

355

Sulphuretted hydrogen..........................

390

Marsh gas.....

403

Sulphurous acid................................

710

Olefiant gas....................................

970

Ammonia...........

1195

Although these gases are perfectly permeable to all the rays of the luminous spectrum, to those of the obscure heat which was employed in these experiments they exhibit a great difference of absorbing power, nitrous oxide gas absorbing 355 times and ammonia 1,195 times as much as dry air. If the tube had been closed with partially athermanous glass instead of dia-thermanous rock salt, no such results could have been obtained, as the glass would have sifted out nearly all the rays of low refrangibility before they fell upon the gases whose powers of absorption were the subject of experiment. - The investigations which have been made upon the subject of diathermancy have been of great advantage in arriving at theoretical conclusions in regard to the molecular constitution of matter. In undertaking to explain why radiant heat of low refrangibility passes so much more readily through elementary than through compound gases, the mind is obliged to form conceptions of the different conditions in which the atoms are arranged in these two classes of matter.

In the elementary gas they must be so disposed as to allow the waves of heat to vibrate freely without accepting their vibrations, while in the compound gas they must be so arranged as to receive or unite with them, or, in common language, to absorb them. In one case, therefore, the mind conceives of the atoms as swinging in the ether singly, receiving but little motion from its vibrations; while in the other they are grouped together in compound masses or molecules, which offer more obstruction to the ethereal waves, and therefore transfer to themselves a corresponding degree of energy. Tyndall found the body ozone to be highly athermanous, a quality which greatly distinguishes it from common oxygen. It has been held that ozone is a compound of oxygen and hydrogen. Now, heat destroys ozone, leaving oxygen; but if it also contains hydrogen, some aqueous vapor must also result from the disorganization of the ozone. This vapor remaining in the oxygen would impair its diathermancy. But the gas obtained by heating ozone is as diathermanous as oxygen obtained in the ordinary way; therefore it contains no aqueous vapor, and the ozone from which it was derived must be simply oxygen, with its atoms grouped together somewhat after the manner of a compound gas.

The diathermancy of iodine to the obscure, and its opacity to the luminous rays, allows the visible to be divided from the invisible spectrum which lies beyond the red rays, by passing the light of an incandescent body through a hollow prism of rock salt containing a solution of iodine in bisulphide of 'carbon. The conclusion to be arrived at from a consideration of this fact is, in the opinion of Tyndall, that the luminous waves which are intercepted by the iodine are in accord with its dissolved atoms, and therefore can transfer their motion or energy to them. Transparency and diathermancy he therefore considers as synonymous with discord, and opacity and athermancy as synonymous with accord, between the waves of ether and those of the molecules of the body on which they fall, or through which they pass. The blackness of lampblack he ascribes to the accord between the vibrations of its atoms and the waves embraced within the luminous portion of the spectrum; and the luminous rays which it absorbs are the ones which it radiates when raised to a sufficient temperature.

But lampblack is also diathermanous to the very extreme obscure rays of the spectrum; a fact which was shown by Melloni. Aqueous vapor, although perfectly transparent to the luminous rays of the spectrum, was found by Tyndall to be quite opaque to those of the dark spectrum. This is one of the most interesting facts connected with the whole subject of heat, and of the greatest importance, not only in a strictly scientific sense, but in its practical bearing upon questions of meteorology, and therefore upon the business of every-day life. The formation of clouds by the radiation and consequent loss of heat from vapor through the drier atmosphere above, as well as by the condensation produced by currents of cool air, and the formation of dew from the same cause, the equableness of moist climates and the cold of high mountains, could never have been well understood unless the subject of the comparative diathermancy of dry and moist gases, particularly of the atmosphere in its various hygrometric conditions, had been carefully investigated.