ME, CHAELES TOMLINSON ON SUPEESATUEATED SALINE SOLUTIONS. 01 
reduced in temperature to about 16° Fahr., there is a deposit of anhydrous salt, as in 
the case of Glauber’s salt and one or two others, and the subsequent behaviour is 
similar. Upon this deposit are built rhomboidal plates of a lower degree of hydration 
than the double salt of the solution. If, now, the cotton-wool be removed and crystal- 
lization sets in from the surface, the fully hydrated salt, on coming into contact with 
the modified salt, renders it opaque without altering its form. If before crystallization 
sets in the upper part of the solution 9 be poured off, and the crystals be taken out and 
dried between folds of filtering-paper, with a view to ascertain their state of hydration, 
they become hot from the fixation of an additional quantity of water, and pass at once 
into the condition of hydration of the normal salt. 
All these phenomena closely resemble the behaviour of a supersaturated solution of 
Glauber’s salt on a proper reduction of temperature. The anhydrous salt passes into 
solution, a portion of it is thrown down when the solution cools sufficiently (and this 
varies with the solubility of the salt and the strength of the solution), and it partially 
enters into solution at the bottom of the vessel ; the heat thus generated further assists 
the solution, until at length a dense solution is formed, occupying the lowest stratum, 
and, in the case of sodic sulphate, containing only water enough to form the 7 -watered 
instead of the usual IQ-watered salt. The crystals of the modified salt thus formed 
occupy the bottom of the vessel, while the solution above is not its mother liquor, as is 
commonly supposed, but still remains a solution of the anhydrous salt, as may be proved 
by again lowering the temperature of the solution, when octahedral crystals of the anhy- 
drous salt will again be thrown down. If at any time after the modified salt has been 
formed the solution be exposed to free air by taking out the cotton-wool plug, crystal- 
lization sets in from the surface, and proceeding downwards carries with it sufficient 
water to convert the 7-atom into the 10-atom salt ; and in doing so it effects such a minute 
state of division as to change the transparent salt into an opaque white. 
It is commonly stated that a supersaturated solution of Glauber’s salt forms the mo- 
dified salt at temperatures between 60° and 40° F. ; but if the solution be clean and con- 
tained in a clean vessel, a highly supersaturated solution, formed of two or three parts 
salt to one of water, may be reduced to 20° and under without throwing down any salt. 
Such a solution contained in an 8-oz. globular flask, the globe of which was quite full, 
was exposed all night to a temperature which fell to a minimum of about 20° F. In 
the morning the solution was perfectly clear and bright, and there was no deposit of the 
modified salt ; but on shaking the flask it instantly became opaque, from the multitude 
of small octahedral crystals that filled the solution ; these quickly subsided, and within 
a quarter of an hour a fine crop of the 7-atom crystals was erected upon them, while 
the solution above still remained that of the anhydrous salt. 
The sulphate of zinc and magnesia is rich in water of crystallization, and hence its 
behaviour is different from that of a double salt containing only a small proportion of 
combined water, such as the double salt formed by dissolving the zincic sulphate in a 
9 This, it should be observed, is not the mother liquor, but a solution of the anhydrous salt. 
