RADIANT HEAT. 87 



action of vapour, very slowly raised from mercury, that 

 the space occupied by the red rays, and those which lie 

 without the spectrum below it, will condense the vapour 

 thickly, while the portion corresponding with the other 

 rays will be left untouched. This affords us evidence of 

 the power of solar heat to produce, very readily, a change 

 in the molecular structure of solid bodies. If we allow 

 the sun's rays to permeate coloured glasses, and then 

 fall upon a polished metallic surface, the result, on 

 exposing the plate to vapourisation, will be similar to 

 that just described. Under yellow and green glasses no 

 vapour will be condensed ; but on the space on which 

 the rays permeating a red glass, or even a blackened one, 

 fall, a very copious deposit of vapour will mark with dis- 

 tinctness the spaces these glasses covered. More remark- 

 able still, if these or any other coloured bodies are placed 

 in a box, and a polished metal plate is suspended a few 

 lines above them, the whole being kept in perfect dark- 

 ness for a few hours, precisely the same effect takes place 

 as when the arrangement is exposed to the full rays of 

 the sun. Here we have evidence of the radiating heat of 

 bodies, producing even in darkness the same phenomena 

 as the transmitted heat-rays of the sun. We must, 

 however, return to the examination of some of these and 

 other analogous influences under the head of actino- 

 chemistry. 



From these curious discoveries of inductive research 

 we learn some high truths. Associated with light 

 oheying many of the same laws moving in a similar 

 manner we receive a power which is essential to the 

 constitution of our planet. This power is often mani- 

 fested in such intimate combination with the luminous 

 principle of the solar rays, that it has been suspected to 

 be but another form of the same agency. While, how- 

 over, we are enabled to show the phenomena of one 

 without producing those which distinguish the other, 

 we are constrained to regard heat as something dis- 



