332 M. Poibson on the Caloric of Gases and Vapours. 



ture any number ot degrees, is proportional to this number, the 

 pressure being constant. Tliis being admitted, if we call C 

 the caloric required to reduce a gramme of water at zero into 

 vapour at 100° and with an elasticity of '"'76; Q the caloric 

 necessary to vaporise this same gramme of water and give it 

 a temperature fl, under any pressure p ; y the same specific 

 caloric of the aqueous vapour under the pressure '" '76; and 

 finally, if we substitute in equation (6) the barometric altitude 

 h for the pressure p, which it measures, this formula will give 



Q = C when A = '"-76 and 9 = 100°, and ^ = y when /i = '»-76. 



d 6 ' 



Determining then in consequence the two arbitrary constants 

 which it contains, it becomes 



Q = C+y[ (266-67+9) (-^) ~ -366-67 ] (8) 



It would be desirable that the accuracy of this formula should 

 be verified by experiment, and the constants C, y, and /c de- 

 termined with precision. 



If we put unity for the specific heat of a gramme of water, 

 or for the quantity of heat necessary to raise its temperature 

 1°, we shall have C = 650 very nearly, by taking the mean of 

 the values found for this quantity by different philosophers. 

 Following MM. Laroche and Berard, we shall likewise have 

 y=-84;7. Indeed they have not given this value of y with 

 much confidence ; but there is reason to believe it is not far 

 from truth, and we shall therefore adopt it until it be modi- 

 fied by other observations. With respect to the value of k 

 we know of no direct observations by which it can be deter- 

 mined ; but an important remark which many philosophers, 

 and particularly MM. Clement and Desormes, have made will 

 enable us to approximate to it. 



According to this remark, when a space is saturated with 

 vapour, the quantity of caloric contained in each gramme is 

 sensibly the same whatever be the temperature ; so that if for 

 5 in the value of Q we put successively different temperatures, 

 and substitute at the same time for h the corresponding maxi- 

 mum tensions of the vapour, Q will be constant or nearly the 

 same in each case. When 9=100°, the maximum tension, 

 h = "'-76, which numbers substituted for 9 and k in the value 

 of Q render the coefficient of y nearly = 0. Consequently de- 

 noting by H instead of h the maximum tension of any tem- 

 perature 9, this coefficient of y must still be nearly=0, what- 

 ever be the value of 9. Hence the following approximate equa- 

 tion : 



(266-67+9) (-^) T- 366-67 = 0; (9) 



from 



