334 Mr Meikle on the Theory of 



tical assumptions. How far I have succeeded will appear from 

 a careful perusal of what follows. 



For this purpose, I shall set out from the same principles as 

 M. Poisson does in his Memoir on the Caloric of Gases and 

 Vapours * ; but it is only for a short way that I can go in with 

 the doctrines which that able mathematician endeavours there 

 to establish ; because, as will shortly appear, his data soon be- 

 come both redundant and inconsistent, though not till he has 

 investigated the law which connects the density and pressure 

 with the temperature by the common scale in a mass of air, 

 when its quantity of heat is constant. This law exactly agrees 

 with that which our learned countryman Mr Ivory has obtained 

 by a different process ; so that no doubt need remain on this 

 part of the subject, so far as mathematical reasoning is concerned. 

 But it is not necessary that I should be first in possession of this 

 law, to establish the law which connects the variations of vo- 

 lume under a constant pressure, with the variations of heat, 

 though indeed they are so intimately connected, that either of 

 them may be deduced from the other. 



The experiments of MM. Gay Lussac and Welter are en- 

 titled to a considerable degree of confidence, and from these it 

 appears, that the specific heat of air under a constant volume, 

 is to its specific heat under a constant pressure, in a ratio sen- 

 sibly constant, viz. that of 1 to 1.375, nearly agreeing with 

 l.S54-(- deduced from the experiments of MM. Desormes and 

 Clement. Adopting this, no objection can be made to M. Pois- 

 son's reasoning till he gets past his equations (5). But imme- 

 diately thereafter, in attempting to prosecute the subject, and 

 supposing his data exhausted, M. Poisson, after the example of 

 the Marquis Laplace, adopts the well known hypothesis already 

 noticed, that the expansions of air under a constant pressure are 

 proportional to the increments of heat ; and it is curious that 

 neither of these distinguished philosophers were aware that this 

 hypothesis was both unnecessary and directly at variance with 

 the above mentioned constant ratio of the specific heats. The 



• Annates de Chim. et de Phys. xxiii. 337. ; PhU. Mag. Ixii. 328. 



•f- Journal de Physique, Ixxxix. 331. The following investigation, so far 

 as regards the law of temperature, has nothing to do with the value of this 

 ratio. 



