"SKETCH OF THERMODYNAMICS" 213 



Sir George Stokes when Mayer was awarded the Copley Medal by the Royal 

 Society of London. Commenting on this determination, Stokes wrote : 



" This was undoubtedly a bold idea, and the numerical value of the mechanical 

 equivalent of heat obtained by Mayer's method is, as we know, very nearly correct. 

 Nevertheless it must be observed that one essential condition in a trustworthy 

 determination is wanting in Mayer's method ; the portion of matter operated on does 

 not go through a cycle of changes. Mayer reasons as if the production of heat were the 

 sole effect of the work done in compressing air. But the volume of the air is changed 

 at the same time, and it is quite impossible to say a priori whether this change may 

 not involve what is analogous to the statical compression of a spring, in which a 

 portion or even a large portion of the work done in compression may have been 

 expended. In that case the numerical result given by Mayer's method would have 

 been erroneous, and might have been even widely erroneous. Hence the practical 

 correctness of the equivalent got by Mayer's method must not lead us to shut our 

 eyes to the merit of our countryman Joule in being the first to determine the 

 mechanical equivalent of heat by methods which are unexceptionable, as fulfilling 

 the essential condition that no ultimate change of state is produced in the matter 

 operated upon." 



Whatever view may be taken of the question, one thing is clear. It 

 was Tyndall's eulogy of Mayer which led to the writing of Tail's Sketch 

 of Thermodynamics. The first and second chapters, to a large extent 

 reproductions of the articles in the North British Review, were printed 

 privately for class use in 1867 under the title Historical Sketch of the 

 Dynamical Theory of Heat; and the book complete in three chapters 

 appeared in 1868. 



When Tait was preparing his Thermodynamics for the press he asked 

 Maxwell for some hints. Maxwell's reply of date Dec. u, 1867, was very 

 characteristic and of great interest as being probably the first occasion on 

 which he put in writing his conception of those fine intelligences Maxwell's 

 Demons as Kelvin nicknamed them who operating on the individual 

 molecules of a gas could render nugatory the second law of thermodynamics. 

 He wrote : 



" I do not know in a controversial manner the history of thermodynamics, that 

 is, I could make no assertions about the priority of authors without referring to 

 their actual works.. ..Any contributions I could make to that study are in the 

 way of altering the point of view here and there for clearness or variety, and 

 picking holes here and there to ensure strength and stability. 



" As for instance I think that you might make something of the theory of the 

 absolute scale of temperature by reasoning pretty loud about it and paying it due 

 honour at its entrance. To pick a hole say in the 2nd law of Acs., that if two 



