194 



we get at once 



HEAT. 



whence 



PC 



ab 



P! 

 a 



v e =3b 



e= 27 6R' 

 Now, if the equation is a correct representation of the behaviour of 



FIG. 115. 



gases and vapours, a can be determined from the change of pressure with 

 temperature of the gas when at constant volume, while b can be deter- 

 mined from the change in pv with change of pressure at constant tempera- 

 ture. R may then be found by inserting any three values of p, v, and 6 in 

 the equation. For details of the calculations we refer to Van der Waals' 

 paper (Physical Memoirs, vol. i., p. 390). He finds for carbonic acid 

 the equation 



/ + 0-OQ874V _ 0.0Q23) = j . OQ646 ^ + ^ 



where the unit of pressure is 1 atmosphere, the unit of volume the 

 volume of 1 gramme at and 1 atmosphere, and t is the temperature 

 centigrade. 



