MR. CHARLES TOMLINSON ON SUPERSATURATED SALINE SOLUTIONS. 663 
is the result of certain conditions which will be examined in the next section. But it 
by no means follows that supersaturated solutions in cooling down undergo any change 
in molecular condition, or produce a modified salt of a lower degree of hydration. Strong 
solutions of many salts may be cooled down below the freezing-point of water, and 
become viscous without crystallizing. For example: — Five parts of sodic acetate to one 
part of water were boiled, filtered, and boiled again at 240°, in a flask, with a thermo- 
meter in the solution, the neck being plugged with lamp-cotton. The solution was left 
in a cold room during about twenty hours, and then placed for some hours in a freezing- 
mixture. The solution remained some time at 14°, when the thermometer being gently 
lifted up several times, without being taken out of the flask, the viscous solution ran off 
from the bulb like a thickish syrup. The thermometer was restored to its place, and a 
wire passed down so as to touch the liquid. It immediately and rapidly crystallized into 
an amorphous mass, and became quite solid ; the temperature rose to 104°, thus showing 
a sudden rise of 90°. 
Now here is a solution which in an open evaporating-dish crystallizes at 150° and 
becomes solid a few degrees below this ; yet it can be cooled down to 14° or lower, 
and be kept in that state for hours without crystallizing. It becomes viscid at this low 
temperature, and so far it undergoes a molecular change ; but this solution has no ten- 
dency, so far as I am aware, to form a salt of modified character. It may be cooled 
at least 140° below its point of crystallization and yet not crystallize, simply because 
there is no nucleus to start the action. In the open evaporating-dish, on the contrary, 
no sooner is the boiling solution in contact with the air, than its surface becomes dotted 
with specks and filaments that are floating about. If these be carefully watched, one or 
more of them will be seen to increase in size by the aggregation of saline particles about 
it, and suddenly from this point fans of crystals begin to radiate. In the meantime 
the sides of the vessel are active in separating the saline molecules, and the more so in 
proportion to the absence of chemical purity. 
The sodio-potassic tartrate also gives remarkable results. Two ounces and a half of 
newly formed crystals were fused in their water of crystallization, one ounce of water 
was then added, and the solution raised to the boiling-point. The solution cooled down 
to the temperature of the air without depositing crystals. The flask was then kept for 
some hours in ice and water, and afterwards in ice and salt until the temperature of the 
solution was 14°. The flask was repeatedly moved round and round in the freezing- 
mixture, but the solution remained perfectly fluid. The flask was now taken out, and 
the thermometer lifted up several times, then quite out of the flask ; the solution was 
also touched with a wire, but crystallization did not set in until the side of the flask had 
been scratched with a wire not chemically clean. (If clean, it would have had no effect.) 
The scratches instantly became chalky white, and crystallization set in very gradually ; 
each crystal as it was formed dragged a viscid line as it slowly descended. There was 
at first but little rise of temperature, but in the course of half an hour the mercury 
gradually rose to 64°, thus showing a rise of 50°. 
