M. Poisson on the Caloric of Gases and Vapours. 331 



known for all pressures, when one of them has been deter- 

 mined for one determinate pressure. Following MM. La- 

 roche and Berard, we have c = -2669 for air under a pressure 

 of "••76, the specific heat of an equal weight of water being 

 unity. Calling therefore P the pressure corresponding to the 

 barometric height "''76, we get 



•2669 = B P* ; 



from which we conclude generally 



c = C2669)(-) *; 



and the value of c^ is deduced from that of c by dividing the 

 latter by k. Since the quantity k exceeds unity, the specific 

 heat of a gramme of air, and generally of any gas whatever, 

 will augment as the elastic force p diminishes. 



If we denote by m the quantity of caloric lost by a gramme 

 of air, when its temperature is diminished 7i degrees, we shall 

 have the pressure p remaining constant, 



m = n (•2669) (^) '"^ 

 For an equal volume, the temperature being invariable, the 

 weight will be - grammes, when the pressure becomes p. 

 Calling therefore m the loss of caloric of this other volume for 

 the same diminution of temperature, we get 



m'= 'LfC2669)(jy""; 



from which we conclude 



"»' _ / 7'' \ i * 



m - Vj) ' (7) 



for the ratio of the quantities of caloric lost by the same vo- 

 lume of air under different pressures. 



§ II. The formulae (6) and (7) are extracted from the 12th 

 book of the Mecanique Celeste. M. Laplace has also extend- 

 ed the former to aqueous vapour. For this purpose he sup- 

 poses, first, that when a gramme of vapour is formed, and 

 neither augmented by more vapour nor diminished by conden- 

 sation, the ratioof its specific caloricunder a constant pressure to 

 its specific caloric under a constant volume is invariable; second- 

 ly, that the quantity of caloric necessary to elevate the tempera- 



* M. Poisson's formula (7) must be regarded as a mere theoretical con- 

 clusion unsupported and even unsanctioned as to numbers by experiments. 

 It is directly at variance with what I have shown, Phil. Mag. vol. Ixu. p. 138, 

 follows from M. Laplace's views. What makes it more curious it is La- 

 place's own conclusion. Such is the unfortunate inconsistency which fol- 

 lows from the doctrine of caloric even in the hands of such men as Laplace 

 and Poisson. — J. H. 



T t 2 tare 



