232 REV. S. M. JOHNSTON 



The hydration theory gives 



c , mW'(AE) 



(1 +11— la) Aw 



where C is the theoretical value of the constant and W' is the amount of active 

 water in the solution at the concentration, that is, the amount of water not in a state 

 of combination with salt. We have, therefore, dividing (2) by (1) 



C'_W' 

 C~W 



w w >_ w(c-g; 



(3). 



Having shown how to calculate the amount of combined water, we shall consider the 

 method of its distribution under the several possible forms of hydration. 



The amount of water which is regarded as combined may be associated either with 

 (1 ) both molecules and ions, (2) with ions only, (3) with molecules only. 



II. (a) To obtain the number of molecules of combined water per molecule or ion on 

 the assumption that both molecules and ions hydrate. 



If W and w be weight of water and of salt added to the water respectively, W 

 the active or uncombined water, a the ionization coefficient at the concentration, 



then — — — is the number of gramme molecules of water present, and w ( " — _' 



is the number of gramme particles (molecules or ions) of salt in solution. 



The ratio gives the number of molecules of water per molecule or ion of salt in 

 solution. The number of molecules of water per molecule or ion of salt dissociated or 

 undissociated, therefore, is equal to 



(w - w y 



I8w(l+(n-l)a)' 



(b) The hydration per ion on the assumption that ions only hydrate is 



(W-W)m 



l&.n.a.w 



(6). 



(c) If the hydration is only molecular, the hydration per molecule is given by the 



formula 



(W-W> - 



18(1 -a)w 



The tables which follow contain the results of computations for ammonium and 

 lithium salts. (When making the observations, 25 and 15 cubic centimetre tubes 

 were used.) 



[Tables. 



