212 PETER GUTHRIE TAIT 



Joule does not seem to have become acquainted with Mayer's essay till 

 about 1850. 



When in 1876 Tail had his attention drawn to Mohr's paper of 1837, 

 he sent a translation of it to the Philosophical Magazine (Vol. n, 5th Series, 

 p. no) and added the following statement : 



About the time when Colding and Joule took up the experimental investigation 

 of Energy at the point where it had been left by Rumford and Davy, there were 

 published a great many speculations as to the nature of Heat and its relation to 

 work. Several of these speculations, especially those of Mayer and S6guin, have 

 been discussed, and at least in part reprinted, in the Philosophical Magazine. It is 

 right, therefore, that the same journal should recall attention to the above paper, 

 which was recently pointed out to me by Professor Crum Brown, and contains 

 what are in some respects the most remarkable of all these speculations. 



Singularly enough, it is not even referred to by Mayer, though his much 

 belauded earliest paper appeared only five years later and in the very same journal. 

 It contains, in a considerably superior form, almost all that is correct in Mayer's 

 paper ; and, though it contains many mistakes, it avoids some of the worst of 

 those made by Mayer, especially his false analogy and his a priori reasoning. 



Polarisation of Heat is ascribed to Melloni instead of Forbes ; the calculation 

 from the compressibility and expansibility of water is meaningless ; and the con- 

 fusion between the two perfectly distinct meanings of the word Kraft is nearly as 

 great as that which some modern British authors are attempting to introduce into 

 their own language by ascribing a second and quite indefensible meaning to the 

 word Force. 



On the other hand, several of the necessary consequences of the establishment 

 of the Undulatory nature of Radiant Heat are well stated ; and the very process 

 (for determining the mechanical equivalent of heat by the two specific heats of air) 

 for which Mayer has received in some quarters such extraordinary praise though 

 it is in principle, albeit not in practice, utterly erroneous is here stated 1 much 

 more clearly than it was stated five years later by Mayer. 



As regards the experimental determination of the dynamical equivalent 

 of heat, Tait's position is practically upheld in the calm judgment given by 



1 Mohr's argument is : " If any species of gas is heated more strongly it strives not only 

 to increase the number of its vibrations, but also to enlarge their amplitudes. If one prevents 

 this expansion, it appears as increased tension. One would require therefore a smaller quantity 

 of heat to warm a gas shut in by firm walls than a gas contained in yielding walls, since, if 

 heat be the cause of the expansion, just as much heat must become latent as there would 

 be cold developed if the gas were allowed to expand as much as before but without supply 

 of heat." In an earlier section he has already stated that " Heat is an oscillatory motion 

 of the smallest parts of bodies.... Heat appears as 'Kraft '...the expansion of bodies by 

 heat is a force-phenomenon of the highest kind." 



