ON THE PHYSICAL VIEW OF NATURE. 



169 



or heat processes and this practically means in all 

 natural processes there is such a quantity which is 

 always on the increase, and which thus measures in 

 mathematical language the growing loss of available 

 or useful energy in the world. Eankine simply called it 

 the " thermo-dynamic function " ; Clausius thought it 

 important to give it a name which would co-ordinate it 

 with energy, and he called it entropy : 1 energy which 

 is turned inside, becomes hidden or locked up. Clausius 

 thus gave a different wording of Thomson's doctrine of 



45. 

 Entropy. 



1 Clausius had already in 1854 

 (Pogg. 'Ann.,' vol. xciii. p. 481) ar- 

 rived at the principal consequences 

 and the final enunciation of what 

 he termed " the second law of 

 thermo - dynamics," a law which 

 refers to the transformation, as the 

 first refers to the conservation, of 

 energy. He there arrives at similar 

 conclusions to those put forth by 

 Thomson two years earlier. The 

 word entropy, however, was not in- 

 troduced by him till 1865 (Pogg. 

 ' Ann.,' vol. cxxv. p. 390), when he 

 introduced it with the following 

 remarks : " I have intentionally 

 formed the word entropy as much 

 as possible on the model of that 

 of energy, for the two quantities 

 which are to be designated by these 

 two words are in their physical 

 meaning so intimately related that 

 a similarity in the terms seemed to 

 me to be justified." As stated 

 above (p. 167, note), Lord Kelvin, who 

 worked simultaneously and inde- 

 pendently at the same subject, laid 

 more stress upon the direct state- 

 ment, that in all transformations 

 of energy we have to distinguish 

 between the available and the total 

 intrinsic energy, and introduced 

 the terms energy and motivity as 



two functions of all the variables 

 specifying the conditions of a 

 system. In his article on Heat, 

 contributed to the 'Ency. Brit.,' 

 9th ed., he gives the mathematical 

 relation of motivity to entropy 

 (' Papers,' vol.iii. p. 167). The term 

 motivity has not become current in 

 thermo - dynamical treatises, but 

 the need has been very generally 

 felt of reserving the word energy in 

 a restricted sense for available 

 energy, such energy as can be put 

 to mechanical use. Wald, in a 

 very interesting dissertation, ' Die 

 Energie und ihre Entwerthung' 

 (Leipzig, 1889), deplores (pp. 43 

 and 44) the fact that the word 

 energy has not been reserved to 

 denote useful, available energy. 

 " Had the word energy," he says, 

 "been introduced before the dis- 

 covery of the first law of thermo- 

 dynamics, then certainly only me- 

 chanical energy would have been 

 termed simply energy." In the 

 use of the word Kraft in some 

 writers, such as Mayer, there 

 seems occasionally a confusion be- 

 tween available and total or in- 

 trinsic energy. See Le Chatelier 

 in 'Journal de Physique,' 1894. 



