314 DISCOURSE ON JHE STUDY 



daced by the concentration of the solar rays by 

 burning glasses, by the combustion of oxygen and 

 hydrogen gases mixed in the exact proportion in 

 which they combine to produce water, and by the 

 discharge of a continued and copious current of 

 electricity through a small conductor. As these 

 three sources of heat are independent of each 

 other, and each capable of being brought into 

 action in a very confined space, there seems no 

 reason why they might not all three be applied at 

 once at the same point, by which means, probably, 

 effects would be produced infinitely surpassing any 

 hitherto witnessed. 



(349.) Heat is communicated either by radiation 

 between bodies at a distance, or by conduction 

 between bodies in contact, or between the conti- 

 guous parts of one and the same body. The laws 

 of the radiation of heat have been studied with 

 great attention, and have been found to present 

 strong analogies with that of light in some points, 

 and singular differences in others. Thus, the heat 

 which accompanies the sun's rays comports itself, 

 in all respects, like light ; being subject to similar 

 laws of reflection, refraction, and even of polariza- 

 tion, as has been shown by Berard. Yet they are 

 not identical with each other ; Sir William Herschel 

 having shown, by decisive experiments, verified by 

 those of Sir H. Englefield, that there exist in a 

 solar beam both rays of heat which are not lumin- 

 ous, and rays of light which have no heating power. 



(350.) The heat, radiated by terrestrial fires, and 

 by bodies obscurely hot, by whatever means they 

 have acquired their heat (even by exposure to the 



