434 Prof. F. Kohlrausch on the Determination of 



we may assume 



| = -0-930; 

 at 



from which the change of pressure till then, due to change in 

 temperature, will be 



-0-75 x 0-939= -0-698 millim. 



This number must be added to the value ?/ = 9 - 912 calculated 

 for 0*75 second, from which the diminution in pressure due to 

 lowering of temperature when the air is rarefied is 



2/ = 10*610 millims. 



From this we get the ratio of the specific heat under constant 

 pressure c to that under constant density c } in the following 

 manner. If the mass of air unity, at the temperature 6, is rare- 

 fied from d to d } without the access of heat from without, it un- 

 dergoes a diminution in temperature of 



1 -\-u6 d — d x c — c l 

 a d c x 



if a. is the coefficient of expansion of gases with the temperature. 

 If the residual pressure after rarefaction, but after restoration 

 of the original temperature, be called p l} the above lowering of 

 temperature produces a diminution of pressure 



Vo=Pi 



d—d { c — c, 



d c, ' 

 or, if p is the pressure before rarefaction, 



P~P\ c ~ c \ 



whence 



1 = 1+ ?/o P - 

 c i P—Pi Pi 



Now in the experiments there was obtained 



p = 752 millims., ^ = 715, ?/ = 10*61 j 

 hence 



.L = 1 + ^l.^ = 1 . 3 02. 

 c l 67 71o 



I have repeated the observations under various conditions 

 — namely with greater and less change of density, with com- 

 pression of the above mass of air instead of rarefaction, with 

 shorter duration of communication (by rapidly opening and clo- 

 sing the stopcock), finally with three different barometers, one 



