ON RADIATION. 459 



development of cold. By placing a drop of chilled water upon the 

 junction of the two metals, Lenz subsequently congealed the water to 

 ice by the passage of the current. 



The source of power in the thermopile is here revealed, and a rela- 

 tion of the utmost importance is established between heat and elec- 

 tricity. Heat is shown to be the nutriment of the electric current. 

 When one face of a thermopile is warmed, the current produced, 

 which is always from bismuth to antimony, is simply heat consumed 

 and transmuted into electricity. 



Loner before the death of Melloni, what the Germans call " Die 

 Identitats-Frage," that is to say, the question of the identity of light 

 and radiant heat, agitated men's minds and spurred their inquiries. 

 In the world of science men differ from each other in wisdom and 

 penetration, and a new theoretic truth has always at first the minority 

 on its side. But time, holding incessantly up to the gaze of inquirers 

 the unalterable pattern of Nature, gradually stamps that pattern on 

 the human mind. For twenty years Henry Brougham was able to 

 quench the light of Thomas Young, and to retard, in like proportion, 

 the diffusion of correct notions regarding the nature and propagation 

 of radiant heat. But such opposing forces are, in the end, driven in, 

 and the undulatory theory of light being once established, soon made 

 room for the undulatory theory of radiant heat. It was shown by 

 degrees that every purely physical effect manifested by light was 

 equally manifested by the invisible form of radiation. Reflection, 

 refraction, double refraction, polarization, magnetization, were all 

 proved true of radiant heat, just as certainly as they had been proved 

 true of light. It was at length clearly realized that radiant heat, like 

 light, was propagated in waves through that wondrous luminiferous 

 medium which fills all space, the only real difference between them 

 being a difference in the length and frequency of the ethereal waves. 

 Light, as a sensation, was seen to be produced by a particular kind of 

 radiant heat, which possessed the power of exciting the retina. 



And now we approach a deeper and more subtile portion of our 

 subject. What, we have to ask, is the origin of the ether-waves, 

 some of which constitute light, and all of which constitute radiant 

 heat ? The answer to this question is that the waves have their origin 

 in the vibrations of the ultimate particles of bodies. But we must be 

 more strict in our definition of ultimate particles. The ultimate par- 

 ticle of water, for example, is a molecide. If you go beyond this 

 molecule and decompose it, the result is no longer water, but the dis- 

 crete atoms of oxygen and hydrogen. The molecule of water consists 

 of three such atoms held tightly together, but still capable of indi- 

 vidual vibration. The question now arises, Is it the molecules vibrat- 

 ing as wholes, or the shivering atoms of the molecules, that are to be 



