ON THE PHYSICAL VIEW OF NATURE. 



175 



It was about this time after experimental research 

 had been carried on for many years by Julius Thomsen 

 and Berthelot, after Horstmann had made a beginning of 



second law of therrno-dynamics can 

 be expressed (' Allg. Chemie,' vol. ii. 

 part 2, p. 150). In every case it 

 is simply a question how most 

 conveniently to express and apply 

 the general principle that heat 

 cannot of itself pass from a colder 

 to a hotter body, the principle on 

 which Fourier built his "The'orie 

 de la Chaleur," and which revealed 

 itself as the rationale of the ex- 

 positions of Carnot when in the 

 middle of the century their hidden 

 truth emerged from the criticisms 

 of William Thomson (Lord Kelvin) 

 and Clausius. Thus already in the 

 different treatment of the same 

 subject there showed itself the 

 twofold tendency which reasoning 

 011 physical matters so frequently 

 exhibits viz., towards physical 

 directness and mathematical ele- 

 gance ; the former leading to prac- 

 tical application, the latter to 

 analytical refinement. Maxwell, 

 in a review of Tait's 'Ther- 

 modynamics,' written in 1877 

 (' Scientific Papers,' vol. ii. p. 

 666), contrasts the methods of 

 Clausius and Thomson, and Prof. 

 Mach ('Warmelehre,' 1896, p. 300) 

 has made similar remarks. Of 

 Thomson the former says, " that 

 he does not even consecrate a 

 symbol to denote the entropy, 

 but he was the first to clearly 

 define the intrinsic energy of a 

 body, and to him alone are due 

 the ideas and the definitions of 

 the available energy and the dis- 

 sipation of energy. ... He avoids 

 the introduction of quantities 

 which are not capable of ex- 

 perimental measurement." Since 

 these criticisms a great deal has 

 been written to make the second 

 law of thermo-dynamics and the 



conception of entropy more intellig- 

 ible. The object here again has 

 been twofold : first, to make the 

 conceptions useful for the practical 

 purpose of perfecting the heat en- 

 gines (Rankine, Zeuner and his 

 school) and of investigating the 

 conditions of chemical equilibrium 

 (Gibbs, Helmholtz, Duhem) ; next, 

 to place the second law, which 

 deals with the transformation of 

 energy, on an equally firm founda- 

 tion with the first law, which 

 deals with the conservation of en- 

 ergy. There is no doubt that the 

 principle of the conservation of 

 energy owes a very large part of 

 its intelligibility to the fact that 

 for purely mechanical systems 

 it follows from such well-known 

 dynamical axioms as the laws of 

 motion. When heat was con- 

 ceived to have a mechanical 

 equivalent in mechanical work, 

 the more general principle of the 

 conservation of energy seemed 

 intelligible by mechanical con- 

 ceptions. The second law, how- 

 ever, introduced a property of 

 natural processes which is not so 

 easily understood mechanically 

 viz., that they are not reversible 

 and this property was shown to 

 be connected with a special phys- 

 ical quantity, for which we have 

 a special sense viz., temperature. 

 The problem of making the second 

 law mechanically intelligible thus 

 coincides with the problem of 

 giving a mechanical definition of 

 temperature. It is not sufficient 

 to call heat a mode (or, more cor- 

 rectly, the energy) of motion ; we 

 must express temperature, on the 

 difference of which the usefulness 

 of heat depends, in some way by 

 motion, we must arrive at a 



