670 MR. CHARLES TOMLINSON ON SUPERSATURATED SALINE SOLUTIONS. 
deposited, and the mother-liquor was simply a solution saturated at the existing tem- 
perature. The cupric sulphate, citric acid, and other salts are examples of this action. 
In some cases supersaturated solutions, if not highly concentrated, may be frozen and 
thawed again, without any separation of the salt ; whereas if such solutions are much 
stronger, they cannot be frozen at temperatures varying from 10° to zero. The cupric 
sulphate is a good example of this. According to Poggiale, 100 parts water at 86° 
dissolve 48-81 parts of the salt ; this is equivalent to 234^ grains of salt to one ounce 
of water. A solution of this kind was put into a freezing-mixture and sank slowly to 
about 16°, when it froze, and the thermometer rose to about 25°. The solidified solution 
now appeared almost white, but on leaving the flask exposed to an atmosphere of 52° 
during some hours, the solution thawed, and recovered its former brightness. A much 
stronger solution was now made (468 grains salt to 1^ ounce water) and filtered into a 
tube that had been washed out with sulphuric acid ; this was repeatedly kept in a 
freezing-mixture at 10° without any deposit of salt. 
V. Anhydrous Salts. 
The salts hitherto treated of in this memoir as presenting the phenomena of super- 
saturation are hydrated, and in general the phenomena are best exhibited in those salts 
which are richest in water of crystallization. Whether anhydrous salts display these 
phenomena in their solutions is a point by no means settled ; for while some observers 
maintain that the solutions of a few anhydrous salts do become supersaturated, others 
deny that such is the case. In perfectly clean flasks, and with clean solutions, I have 
never been able to detect a case of supersaturation in an anhydrous salt. A few examples 
may be given. Two ounces of nitre crystals were added to two ounces of water in a 
clean flask, say 100 parts of nitre in 100 of water, and the solution was raised to the 
boiling-point, filtered into a chemically clean flask, made so by washing it out with 
strong nitric acid. In this flask the solution was boiled again; the neck was then 
plugged with cotton-wool with a thermometer passing through the plug. When the 
temperature of the solution had fallen to about 120°, crystals began to grow up from the 
bottom of the flask. These crystals were built up of short prisms, diminishing in width 
and becoming smaller as they rose into the hotter and weaker part of the solution above. 
Now, according to Gay-Lussac’s Table, 100 parts of water at 131° F. contain 97-05 of 
nitre. My solution contained 100 parts. Supposing that, owing to the absence of a 
nucleus, the solution were supersaturated between the temperatures 131° and 118°, 
it could not remain so in consequence of the cohesive force of the saline particles 
prevailing over the adhesive force of the solvent; it begins to deposit crystals, and then 
the action goes on rapidly ; the temperature rises a few degrees, then declines, and 
when cooled down to the temperature of the air, say 50°, the solution holds about 22 
per cent, of the salt. 
But is there any reason for supposing the solution to be supersaturated \ When 
crystallization begins, it is at the coldest part of the flask that the action commences. 
