Dynamical Theory of Heat. 385 



One point more remains to be considered. Prof. Thomson 

 denies everything to Mayer save a combination of a lucky chance 

 and a false analogy ) and he asks me, do I not know " that even 

 on this point he had been anticipated by Seguin, who, three 

 years before the appearance of Mayer's paper, had obtained 

 and published the same numerical result from the same hypo- 

 thesis V I must frankly confess that I did not know this, nor 

 am I yet aware of it. It would be a source of lively regret to 

 me to find I had done injustice to M. Seguin. There cannot 

 be a doubt that he expressed many correct ideas regarding the 

 transformation of heat into motion in his work Sur V influence 

 des chemins de fer, published in 1839, and which ideas he 

 affirms that he had derived from his uncle, the celebrated Mont- 

 golfier. Knowing this, I stated in the preface to my ' Lectures' 

 that he, M. Seguin, stands "in honourable relationship to the 

 dynamical theory of heat." I was aware that, at page 389 of the 



derivation of the mechanical equivalent of heat from the properties of gases 

 were first clearly laid down by Clausius. In connexion with this point he 

 wntes as follows (Phil. Mag. 1856, vol. xii. p. 242) : — "In the absence of 

 more accurate knowledge, it was formerly assumed, in determining the 

 volumes of the unit of weight of saturated vapour at different temperatures, 

 that vapour, even at its maximum density, still obeys Mariotte's and Gay- 

 Lussac's laws. In opposition to this, I have already shown in my first 

 memoir* on this subject, that the volumes in question can be calculated 

 from the principles of the mechanical theory of heat, under the assumption 

 that a permanent gas when it expands at a constant temperature only ab- 

 sorbs so much heat as is consumed in the external work thereby performed, 

 and that these calculations lead to values which, at least at high tempera- 

 tures, differ considerably from Mariotte's and Gay-Lussac's laws. 



" Even the physicists who had occupied themselves more especially with 

 the mechanical theory of heat did not at that time coincide with this view 

 of the deportment of vapour. William Thomson in particular opposed it. 

 In a memoirf presented to the Royal Society of Edinburgh, and published 

 a year later, in March 1851, he only regarded this result as a proof of the 

 improbability of the above assumption which I had employed. 



" Since then, however, he and J. P. Joule have undertaken to test expe- 

 rimentally the accuracy of this assumption^. By a series of well-contrived 

 experiments, executed on a large scale, they have infact shown that, with 

 respect to the permanent gases, atmospheric air and hydrogen, the assump- 

 tion is so nearly true, that in most calculations the deviations from exacti* 

 tude may be disregarded. With carbonic acid, the non-permanent gas 

 they investigated, the deviations were greater. This is in perfect accord- 

 ance with the remark I made on first making the assumption, which was 

 that the latter would probably be found to be accurate in the same measure 

 as Mariotte's and Gay-Lussac's laws were applicable to the gas. In con- 

 sequence of these experiments, Thomson now calculates the volumes of 

 saturated vapours in the same manner as myself." 



* Poggendorff's Annalen, vol. lxxix. p. 368. Phil. Mag. July 1851. 

 f Transactions of the Royal Society of Edinburgh, vol. xx. part 2. p. 261, 

 X Phil. Trans, vol. cxliii. part 3. p. 357; and vol. cxliv. part 2. p. 321. 



