290 Mr. P. G. Tait on the History of Thermo -dynamics. 



pounded." To this we added, in a note, as follows. u Mayer's 

 method is founded on the supposition that diminution of the 

 volume of a body implies an evolution or generation of heat ; 

 and it involves essentially a false analogy between the natural 

 fall of a body to the earth, and the condensation produced in an 

 elastic fluid by the application of external force. The hypothesis 

 on which he thus grounds a definite numerical estimate of the 

 relation between the agencies here involved, is that the heat 

 evolved when an elastic fluid is compressed and kept cool, is 

 simply the dynamical equivalent of the work employed in com- 

 pressing it. The experimental investigations of subsequent 

 naturalists have shown that this hypothesis is altogether false, 

 for the generality of fluids, especially liquids, and is at best only 

 approximately true for air; whereas Mayer's statements imply 

 its indiscriminate application to all bodies in nature, whether 

 gaseous, liquid, or solid, and show no reason for choosing air for 

 the application of the supposed principle to calculation, but that 

 at the time he wrote air was the only body for which the requi- 

 site numerical data were known with any approximation to accu- 

 racy." To every word of this, with the exception of the word 

 " imply," which is not strong enough, I still adhere. Dr. Tyn- 

 dall's mode of dealing with it is characteristic. He says — 

 " not what Mayer's words f imply/ but what they are " — and 

 then quotes, not from the paper of 1842, to which alone we 

 referred (as the only one which could have a chance of priority 

 over either Joule or Colding), but from a pamphlet published 

 in 1845. 



As to the question which might have arisen between Seguin 

 and Mayer, supposing nothing to have been done in the matter 

 by Davy and Rumford ; everything that was done by Mayer in 

 1842 (I still confine myself to the question of Heat alone) was 

 done by Seguin in 1839. Dr. Tyndall is correct in his remark 

 that Seguin did not, as was originally supposed by Joule, give 

 363 kilogrammetres as the dynamical equivalent. But to say, 

 as Dr. Tyndall does, that " there is no determination whatever of 

 the mechanical equivalent of heat in the above [i. e. Seguin' s] Table/' 

 is simply an error, for Seguin gives all the requisite data, though 

 the thermal unit he employs is by no means convenient. Nothing 

 can indeed be more distinct than his evaluation. An hypothesis 

 explicitly stated by him as to the heat of condensation of vapour, 

 now known to be wrong and to give much less than the true ther- 

 mal effect, rendered the numbers in his Table largely in error. 

 In Seguin' s work we find the following passages : — 

 " Ceci reviendrait a dire que la vapeur n'est que 1'intermediaire 

 du calorique pour produire la force, et qu'il doit exister entre le 

 mouvement et le calorique un rapport direct, independant de 



