ON THE I'HYSICAL VIEW iJi-' NATL'KE. 



175 



It was about this time — after experimental research 

 had been carried on for many years by Julius Thomseu 

 and Berthelot, after Horstmann had made a lieginning of 



second law of therino-dynaiiiics can 

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

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

 is simjjly a question how most 

 conveniently to express and apply 

 the general jiriiiciplc that heiit 

 cannot of itself jiass from a colder 

 to a hotter body, the principle on 

 which Fourier built hi.s "Theorie 

 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 

 dirt'erent treatment of the same 

 subject there showed itself the 

 twofold tendency which reasoning 

 on physical matters so frequently 

 exhibits — viz., towards physical 

 directness and mathematical ele- 

 gance ; the former leading to prac- 

 tical a{)plication, 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 ('Warrnelehre,' 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 

 dehne the intrinsic energy of a 

 body, and to him alone are due 

 the ideas and the definitions of 

 the available euergj- and the dis- 

 sipation of energj'. . . . 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 



concepti(jn of entropy more intellig- 

 ible. The object here again haii 

 been twofold : first, to make the 

 cunceptions useful for the practical 

 jiurpo.-e of perfecting tlie heat en- 

 gines (liiinkine, Zeuaer and his 

 school) and of inve.-^tigating the 

 conditions of cheuiical equilibrium 

 (Gibhs, Helmholtz, Duhem) ; next, 

 to place the second law, which 

 deals with the transformation of 

 energy, on an eijually firm foun<la- 

 tion with the first law, which 

 deals with the conservation of en- 

 ergy. There is no doubt tiiat the 

 jirinciple of tlie consenation of 

 energy owes a very lari;e part of 

 its inti-lligiliility to tlie fact that 

 for purely mechanical i-ystems 

 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 bj- mechanical con- 

 ceptions. The second law, how- 

 ever, introduced a property of 

 natural processes which is not so 

 easily understood n)echanically — 

 viz., that they are not reversible 

 — and tliis property was shown to 

 l)e connected witli a sjiecial phys- 

 ical quantity, for which we have 

 a sjjccial sense — viz., temjierature. 

 The problem of making the second 

 law mechanically intelligible thus 

 coincides with the jiroblem of 

 giving a mechanical definition of 

 temperature. It is not sufficient 

 t<) call heat a mode (or, more cor- 

 rectly, the energy) of motion ; we 

 must ex|iress tcmi>erature, on the 

 dill'erence of which the usefulneta 

 of heat depends, in some way by 

 motion, we must nirivc at a 



