64 MR. CHARLES TOMLINSON ON SUPERSATURATED SALINE SOLUTIONS. 
The leaf-like crystals seem to be deformations of the so-called tetrahedral crystals 
produced by the action of cold on these solutions, highly supersaturated as they were 
at the low temperatures to which they were reduced. From the recurrence of similar 
forms during the solidification of solutions of different salts, it is reasonable to suppose 
that the water of the solution determines the form, and that the presence of the saline 
molecules modifies it. From the abundance of these saline molecules in comparatively 
so small a quantity of water, and in so confined a space as a glass tube, the water as a 
preparatory step to solidification is not able to eject the saline molecules; they there- 
fore remain and influence the forms of the solid which results from the low tempera- 
ture. My business, however, is only incidentally mixed up with the solidification of 
supersaturated solutions. What I wish to insist on is the fact that, in the absence of a 
nucleus, these solutions prefer rather to become solid than to crystallize ; and that there 
is no real freezing (if by freezing is meant, as a necessary condition, the separation of 
salt during the act) is proved, I think, by those cases where the result of a moderate 
elevation of temperature is to produce a bright, clear, and still highly supersaturated 
solution 10 ; while in other cases, where there is a separation of salt, it is the anhydrous or 
a salt of a lower degree of hydration than that employed to make the solution. But if, 
at any time during the process of solidifying or of melting, the solution be subjected to 
the action of a nucleus, crystallization is sure to set in, and the original fully hydrated 
salts are reproduced. 
A solution of the cupric and magnesic sulphates at 4° F. produced the crystals already 
described, and when solid the outside of the tube displayed some elegant forms in dif- 
ferent shades of blue. 
The so die and magnesic sulphates in atomic proportions form, with water, a supersa- 
turated solution ; but on reducing it to low temperatures, the salts are apt to separate. 
When such is not the case, an anhydrous salt is thrown down on greatly reducing the 
temperature ; and as this salt takes up a portion of water, acicular crystals shoot out 
from it. Other interesting phenomena may be observed with solutions of this double 
salt at various degrees of supersaturation and at various temperatures ; as also with the 
sulphates of other bases, such as those of cadmium and of nickel , either alone or in 
conjunction with other sulphates. The addition of potaskic sulphate to other sulphates 
in atomic proportions leads to the formation of double salts, which, so far as I have 
examined them, do not form supersaturated solutions. 
in Rudorhf ( Jahresber . der Chemie, 1862, p. 20) showed that by employing saturated solutions of single salts 
and reducing them to low temperatures, ice is formed, together with a hydrated compound of the salt in ques- 
tion. In my experiments with supersaturated solutions in close vessels, chemically clean, the conditions are, of 
course, quite different, and no ice is formed. In some cases the solutions become viscid, like syrup. 
