20 PHYSICAL PROPERTIES AND COMPOSITION 



means of a reversible cyclic process at constant temperature, 

 performed along c o B F o E c. Since without fall of tem- 

 perature there can be no work produced or consumed, the 

 area of c E o c must be equal to that of B F o B. 



With regard to the characteristic equation in general, we 

 may say that the requisite qualitative agreement with the 

 facts depends on the equation being of the third degree in 

 V, so that equations of the first or second degree in V, like 

 that of Budde (p. 16), are inadmissible as characteristic 

 equations. 



Quantitative test of the characteristic equation. The 

 quantitative test may be carried out in two directions, first 

 as to how far the constants a and b account for the devia- 

 tions from Boyle's law, and then how far the critical data 

 can be calculated from a and b. The following table, refer- 

 ring to ethylene at 20 , allows of a conclusion on the first 

 point ; it is calculated 1 from the equation 



-(F- 0-0024) = 0-0037 (272-5 + 0. 



Here the atmospheric pressure is taken as unit for P, and 

 for V the volume which the given quantity of ethylene 

 takes at o and one atmosphere. 



The conditions for the critical point, i.e. temperature 

 pressure and volume at A in Fig. i, the so-called critical data 

 T k Pk Vk may be calculated by putting the equation in the 

 more general form : 



Baynes, Nature, 22. 186. 



