30 MOLECULAR WEIGHT AND POLYMERISM 



differs only by a vanishingly small amount from the total 

 pressure P. Hence 



P = KP\ and C = ^P N = AVP, 



where k is a constant, equal to 



In this case, then, the concentration of the dissolved 

 substance would not be proportional to the pressure of the 

 gas, but to its square root, which would imply a very wide 

 departure from Henry's law: quadruple pressure would 

 not involve a quadrupling of the concentration, but only 

 a doubling. 



A different molecular constitution, such as N 3 or N 4 , would 

 introduce a corresponding change in the law, so that there 

 only remains to consider the case of hydrate formation, 

 such as N 2 . H 2 O. This case may be treated in a similar 

 manner, by assuming the presence in the gas of an 

 extremely small quantity of the hydrate, or in the solution 

 of, besides hydrate, an extremely small quantity of nitrogen 

 not combined with water. Both assumptions lead to the 

 same result; by assuming a small amount of hydrate in 

 the gas, we bring into play the equilibrium 



N 2 .H 2 0^N 2 + H 2 0, 

 with the condition 



PN 2 H.H 2 O = -2* K s > 



in which PN 2 .H 2 o and P Nj8 are the partial pressures of the 

 hydrate and of free nitrogen respectively. Since obviously 



PN Z = -P -PNaHaO 

 the equation may be replaced by 



-PN 2 .H 2 = &!*> 



where 



Since according to what preceded, the partial pressure of 

 the hydrate is proportional to the concentration of the 



