194 M): Herapath in repli) to Dr. Ajyohn 



gases. This established, the truth of tlie expression S^= 



S. — . rrr, — — , which I have eiven, easily follows from what 



Dr. Apjohn has himself admitted ; for, it being allowed to be 

 true ui gases, it must, from what I have shown, be equally true 

 in vapours over their fluids. 



In other ways the proof of this theorem might be easily de- 

 rived, but I hope what I have said will be sufficient to con- 

 vince Dr. A. that the absurdity of making one constant vary 

 as another constant, entered into no part of my communica- 

 tion. 



Many philosophers appear to have confused themselves by 

 not attending to the manner in which the elasticity of vapours 

 in juxtaposition with their fluids increases ; and hence they 

 have drawn an unwarrantable line of distinction between va- 

 pours and gases. They imagine that the rapid increase of 

 elasticity in vapours, is to be attributed to the increased action 

 alone, which they conceive the heat gives to the vaporous par- 

 ticles ; whereas, if they had onty considered that the specific 

 gravity increases with the elasticity, they would have seen that 

 it is not merely to the augmented action due to the heat, but 

 to this, and the uicreased nimiber of the vaporous particles 

 conjointly, that the great rise of elasticity is owing. By the 

 latter part of his pajier Dr. Apjohn seems to have yielded to 

 the same erroneous ideas, and has by this means near the end 

 of his communication involved himself in an error a little toa 

 obvious, but which I willingly omit to notice out of respect to 

 the candour he has displayed. The same feeling induces me 

 to pass over one or two other thmgs in his paper, which ap- 

 pear to me open to comment. 



Though I have not the shade of a doubt concerning the 

 truth of the formula in Cjuestion, yet I perfectly agree with 

 Dr. Apjohn that experimental proof is desirable ; and hence 

 I should be happy to see such an object effected. But in 

 making these experiments I conceive something of attention, 

 beyond what is needful to secure experimental accuracy, is 

 necessary. For instance, if we set out li'om Gay Lussac's 

 determination of the specific gravity at 212% which Dr. A. 

 seems inclined to recommend, we should be satisfied whether 

 tliis specific gravity was determined from the pressure due to 

 the tension of vapour at 212% or from the pressure due to 

 ebullition at 212*^; for these pressui-es are not equal, as philo- 

 sophers have commonly imaguied, but sensibly different, as I 

 have shown pages 441 and 442, Annals ibr December 1821, 

 and page 27 Annals for January 1822; the former being about 

 tlie l-7th of an inch greater than the latter. Again, it should 



be 



