DIATHERMANCY 81 Melloni's Apparatus. quently 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 inves- tigators (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 ap- paratus used by Melloni in his experiments on the diathermancy of various bodies is repre- sented in the engraving. Nobili's thermo-elec- tric pile, made of alternate layers of bismuth and antimony, represented at a, connected by copper wires with a delicate galvanometer 5, constituted the thermometer employed by Mel- loni to measure the radiant heat transmitted through the substances experimented upon, and which he termed his therrao-multiplier. The transmitted rays were received upon one of the faces of the pile, and generated gal- vanic electricity in proportion to their quan- tity, 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 transmis- sion 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 ascer- taining the deflection of the needle of the ther- mo-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. DIATHERMANCY OF SOLIDS. SUBSTANCES, & of an inch thick. Locatelli lamp. Incan- descent platinum. Copper at 752 F. at^TF. Eock salt Sicilian sulphur 92-3 74 92-8 77 92-3 60 92-8 54 Fluor spar 72 69 42 88 Beryl 54 23 13
Iceland spar . . . 39 28 6
Glass 39 24 6
Rock crystal, clear Smoky quartz Chromate of potash Carbonate of lead . . . 83 37 84 82 28 23 28 28 6 6 15 4 3 3
Sulphate of baryta
Feldspar
24
23
18
19
8
6
18
12
8
o
Selenite
14
5
o Tartrate of potash 11 9 8 2
o
o 8 1 o o Ice 6 0-5
o 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 propor- tionately. Eock salt, it is seen, is equally dia- thermanous to all the sources named in the table. It is found, however, to be not perfect- ly 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 incan- descent platinum, it permits the passage of only 6 per cent, of those which are emitted from copper heated to 752 F., and is com- pletely 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 lo w 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 rela- tions it sustains with organized life. The fol- lowing 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 -^ of an inch in thickness, held in glass cells :
