174 Prof. Tyudall on Mayer, and the Mechanical Theory of Heat. 



of being the experimental demonstrator of the equivalence of 

 heat and work. 



It was not my object in the lecture to which you refer to give 

 a history of the mechanical theory of heat, but simply to place 

 a man of genius, to whom the fates had been singularly unkind, 

 in a position in some measure worthy of him. I was quite 

 aware of all that you have stated regarding Locke, Rumford, 

 Davy, and others : you might have added Bacon to your list. 

 Probably no great generalization was ever established without 

 having first simmered in the minds of many thinkers. But the 

 writings of Mayer form an epoch in the history of this subject ; 

 and I certainly should not feel disposed to retract a single sen- 

 tence that I have written in his favour. I believe he deserves 

 more praise than I have given him. It was he who first used 

 the term " equivalent " in the precise sense in which you have 

 applied it ; he calculated the mechanical equivalent of heat from 

 data which, as I have said, " a man of rare ingenuity alone could 

 turn to account;" and his calculation is in striking accordance 

 with your own experimental determinations*. You worked in- 

 dependently of Mayer, and in a totally different way. You 

 brought the mechanical theory to the test of experiment, and 

 in this way proved its truth. 



Mayer calculated correctly the mechanical equivalent of heat ; 

 but you say that, at the time he wrote, there were no known 

 facts to warrant the hypothesis which he adopted. If by this 

 you mean to say that he made a haphazard guess which had no 

 basis of physical probability, I cannot agree w T ith you. The 

 known constitution of an elastic fluid is, in my opinion, quite 

 sufficient to justify Mayer's proceeding. His hypothesis was 

 this : — Let the quantity of heat required to raise the tempera- 

 ture of gas, preserved at a constant volume, t°, be x, and let the 

 heat required to raise the same gas, under constant pressure, t°, 

 be x-\-y. The weight raised by the expanding gas in the latter 

 case being P, and the height to which it is raised h, then, ac- 

 cording to Mayer, y = -p x h, 



that is to say, the excess of heat imparted in the latter case is 

 precisely equivalent to the mechanical work performed. 



It is undoubtedly implied in this equation that the quantity 

 of heat y is expended wholly in external work, and that none of 

 it has been consumed in overcoming internal molecular at- 

 tractions. This, I think, on the face of it is an extremely pro- 

 bable hypothesis — so probable, indeed, as to amount, in my 

 estimation, almost to a certainty. Clausius makes the same 

 assumption with no better authority than Mayer; and I believe 

 * The corrected specific heat of air being made use of. 



