of Edinburgh, Session 1881-82. 
819 
3. On the Condition of Ammonium Salts when dissolved in 
Water. By W. W. J. Nicol, M.A., B.Sc. Part I. 
When a salt, such as sodium chloride, which crystallises at 
ordinary temperatures without water of crystallisation, is dissolved 
in water, contraction takes place ; that is, the specific gravity of the 
solution is greater than that calculated for a solution of that strength 
from the specific gravity of the salt in the solid state and that of 
water, by the formula — 
100 
P 
d 
= theoretical specific gravity of the solution. 
Where — 
p = the percentage of salt in the solution. 
d — the specific gravity of the solid salt. 
P = the percentage of water. 
D = the specific gravity of water at the temperature of 
observation. 
The amount of this contraction varies greatly, being as much as 
31*2 per cent, in the case of potassium sulphate, and only 2'5 per 
cent, in the case of potassium ferricyanide, when the salt dissolves 
in a nearly saturated solution, the volume of the solid salt being 
taken as 100,* 
The above is the general rule, hut it has been long known that 
ammonium chloride, when dissolved in water, expands instead of 
apparently contracting, consequently the specific gravity of its 
aqueous solution is always less than the theoretical. The amount 
of this expansion varies with the strength of the solution. Sorby 
(i loc . tit.) states that when the solution contains 3 per cent, of the 
salt the expansion is 3*4 per cent., while near the point of satura- 
tion it is 15*8 per cent. 
This apparent expansion of ammonium chloride has been observed 
hv various experimenters, who all agree that it is the only known 
exception to the general rule. Table I. gives the numbers obtained 
by the more important of these, the theoretical specific gravity of 
* Sorby, Proc. Roy. Soc., xii. 544, 
