TAITS "THERMODYNAMICS. 671 



This calculation belongs of course to molecular theory and not to pure 

 thermodynamics, but it shews that we have reason for believing the truth of 

 the second law to be of the nature of a strong probability, which, though it 

 falls short of certainty by less than any assignable quantity, is not an absolute 

 certainty. 



Several attempts have been made to deduce the second law from purely 

 dynamical principles, such as Hamilton's principle, and without the introduction 

 of any element of probability. If we are right in what has been said above, 

 no deduction of this kind, however apparently satisfactory, can be a sufficient 

 explanation of the second law. Indeed some of them have already indicated 

 their unsoundness by leading to determinations of physical quantities which 

 have no existence, such as the periodic time of the alternations of the volume 

 of particular gases'". 



* Szily, Phil. Mag., October, 1876 ; Clausius, Pogg. Ann. CXLII. p. 433 ; Fogg. Ann. CXLVI. 

 p. 585, May, 1872; J. J. Miiller, Pogg. Ann. CLII. p. 105. 



