The reality of the transmission of heat through glass plates, which both Leslie and Brewster appeared to doubt, from the difficulty of distinguishing between heat so transmitted and that which radiates from the.glass after having been for a time absorbed in it, is now sup posed to be sufficiently established by the experiments of NM. Prevost and Do la Roche (` Annals of Philos.; 1803), and also by the researches of 31. Melloni. (' Annales de Chimie,' xlviii., &c.) The first, in order to ascertain the fact, received on a thermometer the rays from a heated body after passing through screens of glass which were renewed no often that they had not time to become heated. The second suffered a thermometer to rise to its maximum by tho rays from a heated body when a transparent glass screen was interposed ; and again, when there was interposed a glass screen blackened so as to prevent the radiant heat from passing, the excess of the rise of the thermometer in the former case above that in the latter gave evidently the effect pro duced by the radiant beat alone. The conclusion at which the experi menters arrive is, that the quantity of heat which radiates through glass is so much the greater as the temperature of the source of the heat is higher. M. Melloni found that of 100 rays incident. on the same plate of glass from an oil-lamp, from red-hot platinum, from copper heated to 734*, and from the same heated to 212°, the numbers were 77, 57, 34, and 12, respectively. lie has also ascertained that all bodies which have the power of transmitting heat are in general more or less transparent ; and that rock-salt is the only known substance in which all the radiant heat falling on it is either reflected or transmitted, whether the temperature of the heated body be low or high.
Diathermacy does not depend upon transparency, either in solids or in liquids. An iron ball heated to 400', placed midway between the blackened balls of a thcrmoscope, will not cause a rise in the liquid, since each bulb receives an equal share of heat ; but if a plate of rock salt he interposed between the iron and one of the bulbs, and a plate of glass of equal thickness between the hot ball and the other bulb, the bulb next the rock-salt will rise in temperature much more rapidly than the one next the glass, although both screens are equal in thick ness, and equally transparent to light. So also, if four liquids equally transparent receive each 100 rays from an argand lamp, water will tranamit only 11, sulphuric acid 17, ether 2I,and oil of turpentine 31; while chloride of sulphur, which is of a reddish colour, allows 63 rays to pass. The diathermacy of different solids and liquids for heat from different sources is stated in it tabulated form in such books as Miller's Chemical Physic's' We do not repeat them here; but state a few of the more modern results given in that work in connection with the subject.
Knoblauch has found that very thin metallic films are diathermic, thus presenting an analogy to their limited transparency to light. Gold and silver transmit certain calorific rays more freely than others, while platinum appears to transmit all the rays with equal facility.
Tyndall has found that the gases exert different degrees of absorptive action on the rays of beat; thus, coal gas Is moro absorptive than atmospheric air, and the vapour of ether considerably more than that of the bisulphide of carbon. Certain rays are more powerfully absorbed by the colourless gases than others. Thus coal gas arrests the heat rays from a source below a visible red heat much more perfectly than it absorbs the rays of the limo-light after they have traversed a thin layer of water.
Diathermacy is not dependent on transparency ; thus black glass and plates of smoked quartz, so opaque that the midday sun is barely visible through them, are much more diathermanous than plates of alum ; and plates of quartz smoked so as to be opaque are more diathermanous than when clean and transparent. Diathermacy seems to be more influenced by mechanical arrangement than by chemical composition. Common table salt is adiathermle : a solution of rock-salt is scarcely superior to pure water in diathermaey, and a solution of alum is equal to one of rock-salt. The latter substance in the solid colourless form approaches perfection in diathermacy as already noticed. But even this has lately been found to absorb certain of the rays of heat somewhat more freely than others. MI other bodies examined by Melloni transmit a quantity of heat which varies with the nature of the source, just as coloured media transmit quantities of light depending on the nature of the colour. Thus if a pencil of solar light fall upon red glass, red rays alone will be trans mitted, the rest being absorbed. This property, whereby different partially diathermanoue media absorb different constituents of the thermic pencil, has been termed therrno-ehrosis or "calorific tint." For as a pencil of solar light incident first on a red plate, and then on one of bluish green, will be totally absorbed, the second plate absorbing what the first transmitted, so a ray of heat may be entirely absorbed by causing it to pass through two media, one of which absorbs the rays transmitted by the other.
By letting the calorific rays pass first through one screen and then through two, M. De In Roche found that, in passing through the second screen, the rays suffered less diminution of intensity than in passing through the first; and the fact is considered as proving that some calorific rays experience more difficulty in passing through glass than others ; consequently that, like light, radiant heat is of different kinds. Ho also observed that a thick plate of glass allows a smaller quantity of radiant heat to pass through than a thin one, and that the difference is so much the less as the temperature of the heated body is higher : and it is inferred that, since radiant heat becomes more capable of penetrating glass as the temperature increases, till the body becomes luminous, heat is only a modification of light.