92 Mr. Ivory's Investigation of the Heat extricated 



Of the two constants a and /3, the first is the well-known 

 expansion of elastic fluids for one degree of the thermometer. 



The fraction -^, and consequently |3, may be found by as- 



certaining the heat disengaged from a given mass of air by 

 a given condensation ; for the proportion of this heat to the 

 heat of temperature required to produce the same condensa- 

 tion, the pressure remaining constant, would be the fraction 

 sought I know not that any such experiment has been made 

 with sufficient precision. It appears difficult to perform it 

 with great accuracy, on account of the small quantity of mat- 

 ter in air when compared with the vessels that contain it and 

 with the thermometer, bulk foi: bulk. But we may employ 

 for the same purpose a very curious and ingenious experi- 

 ment first made by MM. Clement and Desormes, and after- 

 wards repeated by MM. Gay-Lussac and Welter, which as- 

 certains by the variation of the barometer the heat absorbed 

 or exti'icated in the changes of volume. 



Let p, q, T denote the barometric pressure, density, and 

 temperature of a mass of air : then 



p =z cq{\ + ar), 

 c being a given number. Put §' = §(! + a t) ; then g' will 

 be the density of the same mass of air cooled down to zero of 

 the thermometer, the pressure being constant ; and we shall 

 have, p = cq'. (1) 



In this formula we consider f' as a fixed density, and esti- 

 mate all the changes in the condition of the mass of air by 

 means of the variations of the latent heat and the heat of tem- 

 perature. The air being contained in a close vessel, let a small 

 additional portion of air be forced into the vessel : the conse- 

 quent condensation will cause an increase of pressure, an 

 evolution of latent heat, and an equal rise of temperature, all 

 which circumstances are easily expressed by proper changes 

 in equation (1), viz. 



, 1 + aSi 



P + lp = Cq' X -^^^jr. 



After the condensation, the density being fixed, there will be 

 no change in the latent heat ; but the heat of temperature 

 will be dissipated in a short moment of time, and the pressure 

 will decrease a little: let p' be the pressure when the con- 

 densed air has resumed the general temperature, then, 



A communication must now be opened between the confined 



air 



