ISOTHERMAL AND ADIABATIC CHANGES. 



291 



But e+ ye a = yp for air 



.'. 33200 = \/y x pressure -h density 



= Jy 28000 



whence 7=1-406 



2. By comparison of the two elasticities on change of volume of the gas. 



This method was devised by Clement and Desormes. They used a 

 large glass reservoir A (Fig. 167), furnished with a tap B and a water 

 manometer act' to indicate the pressure of the contained air. 



To begin with, A was partially exhausted, and when its contents were 

 at the temperature of the surroundings, the pressure indicated by the 

 level a of the manometer was 

 read. The tap B was now turned 

 on for a moment to establish 

 equilibrium with the external 

 pressure, and then turned off 

 again. The temperature having 

 risen by this compression, which 

 was taken as adiabatic, it was 

 allowed to fall again to its original 

 value, and the final level a' of the 

 manometer was read. 



The experiment may be re- 

 garded as one to determine the 

 ratio of the elasticities. The air 

 originally filling A was regarded 

 as crushed down to a fraction of 

 the volume of A, the same at FIG. 167. 



both readings of the manometer. 



But if we make the same change of volume, first suddenly and adia- 

 batically, and then slowly and isothermally, 



e t sudden increase of pressure 



"y = = - - - . 



' e 9 slow increase of pressure 



Now, in this experiment the volume of the gas is altered by a definite 

 amount suddenly, and the pressure increase noted. The volume being 

 kept the same, the temperature falls to that of the surroundings, and 

 then the increase of pressure above its original value is the same as if 

 the same change of volume had been made slowly, so that 



ab 



Taking one of their results as a specimen 



Atmospheric pressure = 766-51 



7Ko 7 / 1 ULf = itj 



l762'-9 ) rta ' = 10 ' 2 



Initial 

 Final 



, Q . 



=li 



138 

 ~= 1-353. 



