M. Poisson on the Caloric of Gases and Vapours. 337 



known. Thus MM. Laroche and Berard having determined 

 the specific heat of air mixed with vapom- at the temperature 

 39° and pressure '"-76, and moreover knowing the number of 

 grammes of dry air and vapour contained in the mixture, as 

 well as the specific heat of dry air under the same pressure '"-76, 

 have been able to draw from it the sj^ecific heat of vapour at 

 the whole pressure '"'76, and not at the particular tension of 

 the vapour, a case which they have left undecided, Annales de 

 Chimie, tome 85, p. 132. This specific heat of vapour is the 

 value of y, which we have used in the preceding article*. 



Our equation (11) will still hold, if for the specific heats 

 c, c, c", under a constant pressure we substitute the specific 

 heats corresponding under a constant volume. For instance, 

 calling these latter c^, c/, c/' we shall have 

 {n + n) cj' = n c^+n' c,'. 

 Let k, k', Ic" be the several ratios of c to c^, c' to c/, c" to c/', 

 so that c=k c, c' = // c/, c" = k" cj', then from equation (1 1 ) and 

 the preceding we conclude 



, •/ n k c,-|-w' k' c ' _ 



n c"-\-n c" ' 



or if the ratios k, k' are unequal, the quantities r,, c' will, ac- 

 cording to what we have said in § I., be different powers of 

 the pressiu-e p ; from which it results that the ratio k" will not 

 be independent otj}. Thus the ratio of the two specific heats 

 for a constant pressure and volume of the same simple gas 

 being supposed invariable, but different in different gases, can- 

 not be invariable in a mixture of two of more simple gases, or 

 simple gases and vapours. If this ratio has appeared con- 

 stant in the experiments on atmospheric air of different pres- 

 sures, it is because the values of the specific heats for the two 

 component gases oxygen and azote are sensibly the samef. 

 * I cannot satisfy myself of the degree of confidence to be attached to 

 the experiments of MM. Laroche and Berard. Calculations from the in- 

 fluence of currents of air do not impress me with the idea that such me- 

 thods are susceptible of nuich accuracy. Besides, it certainly seems to be ad- 

 verse to the theory of caloric itself, that so rarefied and expanded a body as 

 vapour should have a less specific heat than its generating water ; which is 

 the case in the above philosopher's results. Crawford's method is much 

 more simple and direct, and brings out results more favourable to caloric. 



— J. H. , , . , 



f M Poisson seems here to think the atmosphere a mere mcclianical 

 mixture of oxy-cn and azote. Were this the cage, the proportion ot these 

 elements would scarce!) be so uniformly the same in all jiarts ol tlic at- 

 mosphere as philosophers tell us it is. But Mr. Uarrop s expcrunents 

 namely that nitropcn confined over water absorbs iroin it just as much and 

 no more oxygen than is sufficient to make atmospheric air, appear to put 

 it bcyon.I a doubt that the atmospiiere is a chemical compound, tliougli 

 perhaps but a weak one. — J. H. ,. • 



Vol. G2. No. 307. Nov. 1823. H ii Supposing 



