242 Prof. Clausius on the Ajyplication of the Mechanical 



must undergo condensation; whereas in most works on the 

 steam-engine, amongst others in the excellent work of Do 

 Pambour*, Watt's theorem, that under these eircumstances the 

 vapour remains precisely at its maximum density, is assumed as 

 a fundamental one. 



Further, 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 f 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 ex- 

 pands at a constant temperature only absorbs so much heat as is. 

 consumed in the external work thereby performed, and that these 

 calculations lead to values which, at least at high temperatures, 

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



Even the physicists who had occupied themselves more espe- 

 cially 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 memoir J presented to 

 the Royal Society of Edinburgh, and published a year later, in 

 March 1851, he only regarded this result as a ])roof of ^he im- 

 probability of the above assumption which I had employed. 



Since then, however, he and J. P. Joule have together under- 

 taken to test experimentally the accuracy of this assumption §. 

 By a series of well-contrived experiments, executed on a large 

 scale, they have in fact shown, that, with respect to the perma- 

 nent gases, atmospheric air and hydrogen, the assumption is so 

 nearly true, that in most calculations the deviations from exacti- 

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

 nent gas they investigated, the deviations were greater. This is 

 in perfect accordance 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 ap])licable to the gas. In consequence of 

 these experiments, Thomson now calculates the volumes of satu- 

 rated vapours in the same manner as myself. There is reason 

 to believe, therefore, that the accuracy of this method of calcu- 

 lation will be gradually more and more recognized by other 

 physicists. 



• 

 * ITi^orie des Machines h. Vapeur, par le Comte F. M. G. de Pambour. 

 Paris, 1844. 



t Poggendorff's Annalen, vol. Ixxi^. p. 368; Phil. Mag. July 1851. 

 X Transactious of the Royal Society of Edinburgh, vol. xx. part 2, p. 261 . 

 § Phil. Trans, vol. cxliii. part 3, p. 357 J and vol. cxHv. part 2, p. 321. 



