334 M. Poisson on the Caloric of Gases a7id Vapours. 



the space is not completely saturated, Q, as given by equa- 

 tion (10), will vary more with the variations of /j and 9. The 

 specific heat of vapour depends simply on // ; for denoting this 

 heat by c we have 



c = -847 



(t) 



76 X. -0683 



Dividing this by k or 1 •073, we have the specific heat under a 

 constant volume. 



By means of the value of k we draw from equation (9) 



If this equation was correct, that is, if Q was rigorously 

 constant in the case of saturation, this formula would express 

 in this same case the tension of the vapour in terms of the 

 temperature ; but though Q varies so little, the preceding 

 value of H wanders in high pressures far too much from ob- 

 servations. Thus when 9= 170° H comes out 13 atmospheres 

 instead of 8 ; nor does the formula represent observations but 

 imperfectly in temperatures beneath 100°*. 



Whether the vapour be at a maximum or not, equation (1), 

 which is equally applicable to vapours and gases, will always 

 give the density p of the vapour when the temperature 9 and 

 tension h are known. Therefore calling D the density of the 

 vapour at 100° and under the pressure of '"'76, we obtain 



_D /( 366-67 

 P~ 7-f^' 266-67 + f' 



The weight of a litre f of dry air at the temperature of 100° 

 and pressure of "'"76 is equal to ^"945; and the weight of a litre 

 of vapour | of it or ^•59. Consequently the weight of a volume v 

 of vapour at the temperature 9 and tension /i will be 



V h ,187-g33 



»«-76*266-67-)-^ ' 



the unity volume being the litre. Then calling V the quan- 



* In die Annals of Philosophy for December 1821, 1 have given a theorem 

 which represents experiments within about 2 inches of pressure from 32° 

 to 312°of Fahrenheit, and comprehending a tension from l-5th inch to 167 

 inches, or upwards of 5 atmospheres. It indeed seems to agree with the 

 observations much better than they agree with each other. In fact, I am 

 inclined to doubt the correctness of Dr. Ure's experiments in the higher 

 temperatures. From the manner in which he made them, I think the va- 

 pour of the mercury must have had considerable influence in augmenting 

 the apparent tensions. Probably Mr. P. Taylor's, in the Phil. Mag., vol. ix. 

 page 452, are nearer the truth, though his not describing the manner of 

 his operating * prevents us from using them with that confidence to which 

 they are very likely entitled. — J. H. 



f 61-028 cubic inches, or 21 13 pints. 



• We regret that Mr. P. Taylor's absence from home, and pressing engagements, 

 have as yet prevented his communicating through our pages an account of his ap- 

 paratus. Several of the most eminent men of science both of our own and other 

 countries have examined it. — Edit. tity 



