310 EADIATION. 



take into consideration tlie sliape, size, and complexity of tlio molcculcii by wliicli 

 the ether is disturbed. 



XVI. — SUMMARY AND CONCLUSION. 



Let US now cast a momentary g-lance over the ground that we have left behind. 

 Tlie general nature of light and heat was first brieHy described : the compoiuuling 

 of matter from elementary atoms and the iiifltience of the act of combination on 

 radiation and absorption were considered and experimentally ilkistrated. Tlu-oiigh 

 the transparent elementary gases radiant heat was found to pass as through a 

 vacuum, wliile many of the compound gases presented almost impassable obsta- 

 cles to the calorific waves. This deportment of the simple gases directed our 

 attention to other elementary bodies, the examination of which led to the dis- 

 covery that the element iodine, dissolved in bisulphide of carbon, possesses the 

 })ower of detaching, with extraordinary sharpness, the light of the spectrum from its 

 lieat, intercepting all luminous rays up to the extreme red, and permitting the 

 calorific rays beyond the red to pass freely through it. This substance was then 

 employed to filter the beams of the electric light, and to form foci of invisible 

 rays so intense as to produce almost all the cilects obtainable in an ordinary tire. 

 Combustible bodies were Imrnt and refractory ones were raised to a white heat by 

 the concentrated invisible rays. Thus, by exalting their i-efrangil)ility, the 

 invisible rays of the electric light were rendered visible, and all the colors ot 

 the solar spectrum were extracted from utter darkness. The extreme richness 

 of the electric light in invisible rays of low refrangibility was demonstrated, one- 

 tenth only of its radiation consisting of luminous rays. The deadness of the 

 optic nerve to those invisible rays was proved, and experiments were then added 

 to show that the bright and the dark rays of a body raised gradually to intense 

 incandescence are strengthened together ; that to reach intense white heat intense 

 dark heat must be generated. A sun could not be formed or a meteorite ren- 

 dered luminous on any other conditions. The light-giving rays constitute only 

 a small fraction of the total radiation, their unspeakable importance to us being 

 due to the fact that their periods are attuned to the special requirements of the 

 eye. 



Among the vapors of volatile liquids vast differences were also found to exist 

 as regards their power of absorption. We followed, moreover, various niolecules 

 from a state of li(pu'd to a state of gas, and found, in both states of aggregation, 

 the power of the individual molecules equally asserted. The position of a vapor 

 as an absorber of radiant heat was proved to be detennined by that of the liquid 

 from which it is derived. Reversing our conceptions, and regarding the mole- 

 cules of gases and vapors not as the recipients, but as the originators of wave 

 motion — not as absorbers, but as radial(jrs — it was proved that the poweis of 

 absorption and radiation went hand in hand, the self-same chemical act which 

 rendered a body competent to intercept the waves of ether rendering it compe- 

 tent in the same degree to generate them. Perfumes were next subjected to 

 examination, and, notwithstanding their extraordinary tenuit\% were found vastly 

 superior, in point of absorptive power, to the body of the air in which they were 

 diffused. We were led thus slowly up to the exannnation of the most widely 

 diffused and most important of all vapors — the aqueous vapor of our atmos- 

 })here — and wo find in it a potent absorber of the purely calorific rays. The 

 })ower of this substance to influence climate, and its general influence on the 

 temperature of the earth, were then briefly dwelt upon. A cobweb spread above 

 a blossom is sufiicient to protect it from nightly chill ; and thus the aqueous vapor 

 of our air, attenuated as it is, checks the drain of terrestrial heat and saves the 

 surface of our planet from the refrigeration which would a,ssuredly accrue were 

 no such substance interposed between it and the voids of space. We considered 

 the influence of vibrating period and molecular form on absorption and radiation, 



