Page:The American Cyclopædia (1879) Volume VI.djvu/91

DIATHERMANCY DIATOMACE^E 83 previous experiments were avoided. The fol- lowing 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 pos- sess ; the transmission was made through each of the gases at the common pressure of the atmosphere : DIATHERMANCY OF GASES. Air Oxygen Nitrogen Hydrogen Chlorine Hydrochloric acid Carbonic oxide Carbonic acid Nitrous oxide Sulphuretted hydrogen. Marsh gas Sulphurous acid defiant gas Ammonia Relative absorption. 1 1 1 1 89 62 90 90 855 390 403 710 970 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 dif- ference 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 con- clusions in regard to the molecular consti- tution 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, re- ceiving 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 correspond- ing 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 com- pound 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 ; there- fore it contains no aqueous vapor, and the ozone from which it was derived must be simply oxygen, with its atoms grouped toge- ther somewhat after the manner of a com- pound gas. The diathermancy of iodine to the obscure, and its opacity to the luminous rays, allows the visible to be divided from the in- visible spectrum which lies beyond the red rays, by passing the light of an incandescent body through a hollow prism of rock salt con- taining 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 black- ness 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 lamp- black is also diathermanous to the very ex- treme 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 con- nected 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 at- mosphere in its various hygrometric condi- tions, had been carefully investigated. DIATOMAEJ3, minute plants growing in moist situations, in collections of fresh water or in the sea, consisting of frustules of various forms, the walls of which contain a large quantity of silex, and are often beautifully diversified and marbled by striae or by dots. Notwithstand- ing the general resemblance of these curious vegetations to the species of desmidiera, they