encountered in the Stud// of Thermodynamics, 211 



words of one syllable, and I believe that at present that is a 

 good thing- to do. Less rudimentary statements of principles 

 may come all in good time, just as in time Hertz's principles 

 of mechanics may oust Newton's laws from the position they 

 still hold as the basis not only for teaching but, what is more 

 important, tor learning the principles of dynamics. 



I propose, then, that instead of inserting new substance 

 into the statements of the second law, we leave the second 

 law alone and put our substance into independent form as a 

 new bypothesis. It will be an hypothesis only in the general 

 sense in which any induction is hypothetical ; it is really a 

 conclusion, even though an indirect one, from a great accu- 

 mulated mass of experience, and it is based on evidence of 

 the same nature as the second law or the principle of the 

 conservation of energy, though the evidence is not so ex- 

 tensive. The particular form we select for the new statement 

 seems to me of small importance, — probably it would be best 

 to start as earlv as possible and say that for all irreversible 



cycles I -q is less than zero and not equal to zero. But 



this leads me to Professor Orr's last point, namely his plea for 

 the use of cycles wherever they can be used, instead of 

 dragging in entropy, or still worse, free energy and thermo- 

 dynamic potentials. 



This may be looked at from various points of view. In 

 the first place, are we speaking of a journal article, of a text- 

 book, or of a treatise ? The object of a journal article is, in 

 general, to influence as many people as may be, and to that 

 end many people must read it. It is unquestionable that 

 many men will read thermodynamics put in the form of 

 cycles who would not read it if entropy or free energy were 

 used in the reasoning. So far, then, as journal articles are 

 concerned. I agree with Professor Orr as to the undesirability 

 of using the more modern forms of the theory when they are 

 not strictly necessary. 



But suppose that we are considering how a tex^-book 

 should be written. Let us grant at once that in an elemen- 

 tary book it is unwise to pile any further abstractions upon 

 Carnot's cycle ; we do not, therefore, admit the same propo- 

 sition for a more advanced text-book. Thermodynamics 

 .-;eems to me to be a very difficult and abstract subject, and 

 one only to be pursued with advantage by students who have 

 already attained a considerable maturity of mind beside 

 having been familiar with mathematical modes of thinking 

 For several years. To such students, the mere mathematical 

 difficulties encountered in dealing with free energy or the 



P2 



