256 Professor Tyndall [March 16, 



electricity. Heat is shown to be the nutriment of the electric 

 current. When one face of a thermopile is warmed, the current pro- 

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

 consumed and transmuted into electricity. 



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

 Identitiits 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 people 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. 

 Eeflection, 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 realised 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 subtle 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 particle of water, for example, is a molecule. If you go 

 beyond this molecule and decompose it, the result is no longer water, 

 but the discrete atoms of oxygen and hydrogen. The molecule of 

 water consists of three such atoms tightly held together, but still 

 capable of individual vibration. The question now arises : Is it the 

 molecules vibrating as wholes, or the shivering atoms of the mole- 

 cules that are to be considered as the real sources of the ether waves ? 

 As long as we were confined to the experiments of Leslie, Eumford, 

 and Melloni, it was difficult to answer this question. But when it 

 was discovered that gases and vapours possessed — in some cases to an 

 astonishing extent — the power both of absorbing and radiating heat, a 

 new light was thrown upon the question. 



