530 Scientific Proceedings, Royal Dublin Society. 
during many years past, I have been accustomed, since 1882, to 
explain the cause of the phenomena by stating that the salt in 
solution is not that which crystallizes out suddenly; that which 
is formed in the solution by the action of heat is not the anhy- 
drous compound, but a hydrated salt containing fewer molecules 
of water than the salt which was dissolved. It is essential that 
it should be more soluble at the higher temperature. ‘The follow- 
ing are some of the facts which have led to this conclusion. 
Anhydrous salts much more soluble in hot than in cold water, 
such as potassium nitrate and silver nitrate, were not found to 
yield supersaturated solutions capable of sudden crystallization. 
If they are supersaturated when first cooled, such supersatura- 
tion is very slight, and the excess of salt in solution is gradually 
deposited, and is complete in twenty-four hours. 
Soluble hydrated salts which have been completely dehydrated, 
such as sodium sulphate and manganese sulphate, if put into water, 
do not dissolve until they have first become hydrated again; the 
former salt then becomes the soluble hydrate with 10H.O, the 
latter the hydrate soluble, in one-third its weight of water, with 
7H,0. When moistened with an insufficiency of water to form 
a solution, or even with an excess of water, they solidify to hard 
solid masses, apparently insoluble in water at 16°C.: this is parti- 
cularly the case with the manganese salt. These hard masses only 
gradually become further hydrated and dissolved. A mixture of 
anhydrous manganese sulphate, with from 60 to 70 per cent. 
coarsely ground ferric oxide, sets quite hard, like Portland cement, 
and may remain in this condition under water without sensible 
diminution in quantity by solution, the hydration and sub- 
sequent solution being a slow process at the ordinary tempera- 
tures. 
There are, in fact, several salts which, in the anhydrous condi- 
tion, afford no evidence of being soluble in water to any extent 
while remaining in that state, though when they have once entered 
into combination with water they become extremely soluble. In 
confirmation of the statement made with respect to sodium sulphate 
let me refer to a classical memoir of M. H. Le Chatelier.: 
1* Recherches Expérimentales sur la constitution des matiére hydrauliques.” 
Extrait des Annales des Mines. May and June, 1887. Paris Vue. Ch. Dunod, 1887. 
