TAITS "THERMODYNAMICS. 669 



" It is impossible, by means of inanimate material agency, to derive mechanical effect from any 

 portion of matter by cooling it below the temperature of the coldest of surrounding objects." 



Without some further restriction this axiom cannot be considered as true, 

 for by allowing air to expand we may derive mechanical effect from it by 

 cooling it below the temperature of the coldest of surrounding objects. 



If we make it a condition that the material agency is to be left in the 

 same state at the end of the process as it was at first, and also that the 

 mechanical effect is not to be derived from the pressure of the hot or of the 

 cold body, the axiom will be rendered strictly true, but this brings us back 

 to a simple re-assertion of Carnot's principle, except that it is extended from 

 heat engines to all other kinds of inanimate material agency. 



It is probably impossible to reduce the second law of thermodynamics to 

 a form as axiomatic as that of the first law, for we have reason to believe 

 that though true, its truth is not of the same order as that of the first law. 



The first law is an extension to the theory of heat of the principle of 

 conservation of energy, which can be proved mathematically true if real bodies 

 consist of matter "as per definition," acted on by forces having potentials. 



The second law relates to that kind of communication of energy which 

 we call the transfer of heat as distinguished from another kind of communication 

 of energy which we call work. According to the molecular theory the only 

 difference between these two kinds of communication of energy is that the 

 motions and displacements which are concerned in the communication of heat 

 are those of molecules, and are so numerous, so small individually, and so 

 irregular in their distribution, that they quite escape all our methods of obser- 

 vation ; whereas when the motions and displacements are those of visible bodies 

 consisting of great numbers of molecules moving altogether, the communication 

 of energy is called work. 



Hence we have only to suppose our senses sharpened to such a degree 

 that we could trace the motions of molecules as easily as we now trace those 

 of large bodies, and the distinction between work and heat would vanish, for 

 the communication of heat would be seen to be a communication of energy of 

 the same kind as that which we call work. 



The second law must either be founded on our actual experience in dealing 

 with real bodies of sensible magnitude, or else deduced from the molecular theory 

 of these bodies, on the hypothesis that the behaviour of bodies consisting of 

 millions of molecules may be deduced from the theory of the encounters of 



