DIA.THERMANCY, a word used to express that quality in bodies by which rays of heat are allowed to pass through them; in other words, it may be called " trans parency" to heat. More correctly speaking. D. has the same relation to radiant heat that transparency has to light. Bodies which have the property of D. arc called diathermanous bodies. The earliest observers of the phenomena of D. supposed that the diathermanous body absorbed the rays of heat and then gave them out; but the fallacy of this idea was shown by Prevost, who ignited substances by rays of heat after passing them through ice. The phenomena of D. have, however, been more extensively examined by 3Ielloni, Tyndall, Bunsen, Kirchhoff, and Balfour Stewart. The experi ments of Melloni, who was the pioneer in the investigation, were of a brilliant character, and the apparatus used by him is a good example of that which has been used so effectively of late years in the rapid advancement of modern science. The thermo electric pile of Nobili was employed to measure the rays of heat, and compare those which were passed through different substances with those passed through air alone. He found, among other things, that rock-salt was almost perfectly diathermanous; there was a loss of only 7.7 per cent, which he attributed to reflection. The following table shows the D. of various solids, and also that their D. varies with the intensity of the heat which is the source of the rays. The substances were yv of an in. thick.
The equal D. of rock-salt to the rays of heat of different degrees of intensity as shown in the above table was taken by Melloni as evidence that D. was perfect. Rays of very low refrangibility, however, will not all pass through rock-salt. Fluor spar is seen to be comparatively diatheomanous, but Iceland-spar and glass show very great variation in D. to rays of different intensity. Feldspar does not vary much in property from them, but alum and ice are almost completely-atheomanoes. The follow ing list shows the percentage of rays transmitted through liquids, held in glass cells, of inch thick :—Bisulphide of carbon, 63; bichloride of sulphur, 63;. oil of turpentine, 31; olive-oil, 30; naphtha, 28; oil of lavender, 26; sulphuric ether, 21; alcohol, 15; acetic acid, 12; distilled water, 11.
After rays of heat have been passed through a body, it will then pass through a second body of the same kind, a phenomenon which might be expected from the nature of radiation. The rays being already sifted, and those transmitted which the molecular structure of the body will allow to pass, they possess the wave-lengths required to make their way between the molecules of the medium. Another quality rather more remark
able, which bodies possess, is, that they are atheomanous to rays of heat which are gen erated in the same substance. Thus rock-salt is nearly athcomanous to heat radiated by rock-salt. The absorbing and radiating powers of bodies are reciprocal and equal; therefore the D. of a body is inversely proportional to its radiating power. The inves tigations of Tyndall on the D. of various liquids and gases possess great interest. IIe found that as a rule elementary bodies were more diatheomanous than compound bodies. A solution of iodine in bisulphide of carbon placed in a prism of transparent rock-salt was found to transmit 29 per cent of all the rays of beat emitted by a non-luminous body. Elementary gases and their mixtures were found almost perfectly diatheoman ous, while many compound gases were diatheomanous only to rays containing much light. The amount of heat transmitted by nitrous oxide gas is only of that which will pass through dry common air, a proof that the atmosphere is merely a mechanical mixture. The following list shows the relative absorbing power of various gases, or their D. in inverse proportion: Air, 1; oxygen, 1; nitrogen, 1; hydrogen, 1; chlorine, 39; hydrochloric acid, 62; carbonic oxide, DO; carbonic acid, DO; nitrous oxide, marsh gas, 403; sulphurous acid, 710; oleflant gas, 790; ammonia, 1195. These gases are all perfectly diatheomanous to luminous heat. The above numbers were obtained only with heat of low refrangibility. These gases were held in tubes of rock-salt. If glass had been used, it would already have sifted out all the rays to which these gases are atheomauous, and the results above indicated would not have been obtained. Ozone, which is an allotropic condition of oxygen (q.v.), was found to be very atheomanous. But on heating the ozone it became perfectly diatheomanous like oxygen. This proves that ozone is not-, as some have supposed, a compound of oxygen and hydrogen, because the heating would produce some vapor, and this would prevent the contents of the tube from being diatheomanous, for aqueous vapor was found to be quite opaque to the dark rays of the spectrum, although diatheomanous to the luminous rays. The subject of D. has much interest in relation to the study of meteorology, and the effects of moisture on climate.