182-184] Thermodynamics 161 



Incidentally we notice that by comparing the two we obtain for the 

 internal pressure pi the value 



(386). 



Returning to the values just found for P, and replacing P by its known 

 value 



we obtain 



_vb(po)-vb(p) ,oo 7 v 



^~ -2F~ ........................ (387) - 



We also have found that 



hence, by subtraction, 



-.-^^ ........................... (388). 



These equations give OT; and S7 C separately. 



184. To obtain some insight into the meaning of these pressures, let us 

 examine what they become when the deviations from Boyle's Law are small. 



If we put p = -^j^ in Van der Waals' equation, we get 



or again, for the pressure on an area inside the gas, noticing that the 

 cohesion term must be supposed to vanish in this case, 



From these equations, we have, on replacing RT by ^T , 



N 



v b 



so that 



Thus r f , as we should expect, becomes identical, except as regards sign, 

 with the cohesion pressure p l assumed in arriving at Van der Waals' equa- 

 tion, and 



_ v b (p) - v _ v_ (_v __ J _ vjb- 

 2h ~2h\v-b }~2hv' 

 j. 11 



