294 Prof. Tyndall on the Influence of Colour 



phorus soon fuses and ignites ; its ignition, however, is not en- 

 tirely due to radiant heat, but mainly to the heat imparted to it 

 by the glass*. 



The fusing-point of phosphorus is about 44° C, that of sugar 

 is 160°; still at the focus of the electric lamp the sugar fuses 

 before the phosphorus. All this is due to the diathermancy 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 con- 

 densed 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 tire. "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 sum- 

 mits of the mountains; for the luminous rays penetrate the tro- 

 pical 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 ex- 

 posed 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 pa- 

 ramount 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 con- 



* I believe this deportment of phosphorus towards radiant heat is not 

 unknown to chemists. 





