Page:The American Cyclopædia (1879) Volume VI.djvu/90

 82 DIATHERMANCY DIATHERMANCY OP LIQUIDS. LIQUIDS, ft of n Inch thick. Percentage of tranuniuion. 68 68 62 81 80 28 26 21 A Imhol 15 12 12 Distilled water 11 A remarkable fact in relation to the diather- mancy 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 diatherma- nous to all sources of heat, absorbs most of the rays that are radiated by beated 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 bn 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 ab- sorbed ; 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 diather- mancy of a body may therefore be stated as inversely proportional to its power of radia- tion. Athermanous bodies, or those which are only slightly diathermanous, are more per- meable 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 in- creased. The investigations of Tyndall on the heat-absorbing powers of various liquids and gases, or in other words their relative diather- mancy, have also thrown much light on the sub- ject of the molecular constitution of matter. He has shown that elementary bodies are general- ly 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 luminous- ness. Converging the rays which were trans- mitted 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 me- chanical mixtures he found to be almost per- fectly 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 propor- tion ; and this he regards as one of the strong- est proofs that the atmosphere is a mechanical mixture, and not a chemical compound. In experimenting upon the conductivity of dif- ferent substances, Tyndall found that this property in a body was generally commensu- rate 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 Tyn- dall 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 boiliog water, or to a temperature far below redness, and the rays were passed through a tube whose ends were closed with plates of trans- parent 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