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XLII. 
A THEORY OF THE MOLECULAR CONSTITUTION OF 
SUPERSATURATED SOLUTIONS. By W. N. HARTLEY, 
F.R.S. 
[Read May 22; Received for Publication May 31 ; 
Published Aveust 3, 1901.] 
Tue salts which ordinarily are capable of forming supersaturated 
solutions have been ascertained to possess the following char- 
acters :-—Ist, they contain several molecules of water of ecrystal- 
lization, and form several different hydrates; 2ndly, they are much 
more soluble in hot than in cold water; 3rdly, they are capable of 
being suddenly solidified as a mass of crystals. For examples we 
have the following :--Na,SO,10H.,O, and the corresponding 
selenate, Na,CO;10H.O, Na,HPO,12H.0, and the correspond- 
ing arseniate, Na,B,O;10H,O0: the alums, as for instance, 
Al.380.24H.0, Na.S,0,5H.0, and NaC,H,;0,3H,0. 
The formation of solutions of these salts by heating the solids 
with a minimum of water, their preservation by protecting them 
from dust, and their sudden crystallization accompanied by rise of 
temperature are well-known phenomena. For their solidification 
either a fragment of a crystal of the same salt, or one of the 
similar constitution and isomorphous with it, must be introduced 
into the hquid. Their behaviour has been carefully investigated 
by J. Millar Thomson.! 
The anhydrous compounds will not cause crystallization ; 
neither will a hydrate containing fewer molecules of water than 
belongs to the original salt. It is well known also that, if a 
solution of sodium sulphate which is supersaturated be cooled 
for some hours below 15° C., say at 10°, it deposits a small pro- 
portion of the salt with seven molecules of water instead of ten. 
From these and other facts which have come under observation, 
1 Trans. Chem. Soc., 35, 1879, pp. 196-206. Also Thomson and W. P. Bloxam, 
Trans. Chem. Soc., 41, 1882, pp. 879-887. 
