ME. CHAELES TOMLINSON ON SUPEESATUEATED SALINE SOLUTIONS. 667 
sequent on the cooling, or on the separation of a fragment of cotton-wool, and then the 
whole solution immediately became solidified into the 10-atom salt. The deposit of a 
few anhydrous crystals, as stated above, is, I think, a sufficient proof that the- solution 
undergoes no molecular change at 60° or under, but continues to be a solution of the 
anhydrous salt until, by a proper reduction of temperature, it deposits the excess that it 
can no longer hold. 
That which takes place in the case of the zincic sulphate solution applies equally here. 
The anhydrous sodic sulphate thus thrown down at 32° liberates heat, and the crystals 
partially enter again into solution. Or should the atmospheric temperature rise a few 
degrees, the operation is further assisted; the anhydrous crystals become wholly or 
partially dissolved, and a dense lower stratum is thus formed, containing not water 
enough for the formation of the 10-atom hydrate, but only sufficient for the 7 -atom; 
and thus we have a comparatively small group of crystals of the 7-atom hydrate formed 
at the bottom of the solution. 
In this way, as it appears to me, the formation of the 7-atom hydrate may be accounted 
for and rendered intelligible without having recourse to the doctrine of molecularity of 
which we really know so little. The mother-liquor resting on this 7-atom hydrate is still 
a supersaturated solution ; so that if the flask be opened crystallization proceeds rapidly 
from the surface downwards, and carries down sufficient water to form the 10-atom salt 
throughout, and also to convert the 7-atom hydrate into the 10-atom ; and in appropri- 
ating the three additional equivalents of water, the 7-atom hydrate becomes of an opaque 
white, from the breaking up of the mass into a multitude of minute 10-atom crystals. 
It is important to bear in mind the difference of locality in the formation of the two 
hydrates; the 7-atom hydrate originates at the bottom of the solution under circum- 
stances which do not allow of sufficient water being present to form a salt of a higher 
degree of hydration than the 7-atom ; whereas the 10-atom salt originates at the surface, 
and carries down in close well-packed crystalline lines sufficient water to form the 
10-atom salt. These lines form so quickly, and are packed so closely together, that 
there is neither time nor space for the formation of the usual crystal ; they pass to the 
very bottom of the vessel, and seem to penetrate while they render opaque the transpa- 
rent crystals formed there. 
M. Lowel states that supersaturated solutions of the sodic carbonate form two modi- 
fied hydrates, viz. the 7HO a and the 7HO h , which, he says, not only differ in solubility 
from each other, but also from that of the 10-atom salt. He admits that the sodic car- 
bonate differs in behaviour from the sodic sulphate, since the carbonate does not deposit 
crystals from the supersaturated solution merely by a reduction of temperature ; “ but 
such solutions remain saturated until, in consequence of lapse of time, and above all of 
low temperature, the inner surfaces of the flask have recovered their peculiar catalytic 
properties of determining crystallization which heat had deprived them of, the cause of 
which remains unknown.” In chemically clean vessels I have not been able to repro- 
duce M. Lowel’ s results. I have tried numerous experiments with solutions containing 
mdccclxviii. 4 Y 
