AND MECHANICAL CONDITION ON EADIANT HEAT. 85 



thermancy of the phosphorus. A thin disk of the substance placed between two plates 

 of rock-salt permits of a copious transmission. This substance therefore takes its place 

 with other elementary bodies as regards deportment towards radiant heat. 



The more diathermic a body is, the less it is warmed by radiant heat. No perfectly 

 transparent body could be warmed by purely luminous heat. The surface of a vessel 

 covered with a thick fur of hoar frost was exposed to the beam of the electric lamp 

 condensed by a powerful mirror, the beam having been previously sent through a cell 

 containing water ; the sifted beam was powerless to remove the frost, though it was 

 competent to set wood on fire. We may largely apply this result. It is not, for 

 example, the luminous rays, but the dark rays of the sun which sweep the snows of 

 winter from the slopes of the Alps. Every glacier stream that rushes through the 

 Alpine valleys is almost wholly the product of invisible radiation. It is also the 

 invisible solar rays which lift the glaciers from the sea-level to the summits of the 

 mountains ; for the luminous rays penetrate the tropical ocean to great depths, while 

 the non-luminous ones are absorbed close to the surface, and become the main agents in 

 evaporation. 



It is often stated, without limitation, that ether might be exposed at the focus of a 

 concave mirror without being sensibly heated; but this can only be true of a sifted 

 beam. At the focus of the electric lamp, not only ether, but alcohol and water are 

 speedily caused to boil, while bisulphide of carbon, whose boiling-point is only 48° C, 

 cannot be raised to ebullition. In fact exposure for a period sufficient to boil alcohol 

 or water is scarcely sufficient to render bisulphide of carbon sensibly warm. 



If any one point came out with more clearness than any other in my experiments on 

 gases, liquids, and vapours, it was the paramount influence which chemical constitution 

 exerted upon the phenomena of radiation and absorption. And seeing how little the 

 character of the radiation was affected by the change of a body from the state of vapour 

 to the state of liquid, I held it to be exceedingly probable that even in the solid state 

 chemical constitution would exert its power. But opposed to this conclusion we had 

 the experiments of Melloni on chalk and lampblack, and the far more extensive ones 

 of Masson and CouRTfiEP:fiE on powders, which seemed clearly to show that in a state of 

 extremely fine division, as in chemical precipitates, the radiant and absorbent powers of 

 aU bodies are the same. From these experiments it was inferred that the influence of 

 physical condition was so predominant as to cause that of chemical constitution to dis- 

 appear *. 



A serious oversight, however, seems to have connected itself with all the experiments 

 of these distinguished men. Melloni mixed his lampblack and powdered chalk with 

 gum or glue, and applied them by means of a camel's-hair brush on the surfaces of his 

 radiating cube. Masson and Court£epee did the same. Melloni, it is true, thus com- 



♦ Masson and CouET^EPiiE, Comptes Eendus, vol. xxv. p. 938 ; J amis, Coutb de Physique, vol. ii, p. 289. 



