122 SCIENCE: PROGRESS 
causes crystallisation to begin, and the same effect may often 
be produced by shaking. 
Break thou deep vase of chilling tears 
That grief hath shaken into frost, 
are well-known lines from /z Memoriam that refer to this 
remarkable effect. 
It is curious that no systematic observations have hitherto 
been made upon the behaviour of solutions which are con- 
tinuously shaken while cooling. 
In the course of some experiments upon the refractive 
indices of cooling supersaturated solutions of various salts 
made by Miss Florence Isaac and myself in the Mineralogical 
Laboratory at Oxford we noticed that when such a solution 
is stirred in an open vessel, a thin shower of crystals generally 
appears at or about the saturation temperature; but that after 
the temperature has fallen about 10° below this point there 
ensues a dense shower of crystals, which is generally so thick 
as to make the liquid almost opaque. The sudden weakening 
of the solution at the lower temperature is accompanied by a 
sudden fall of the refractive index, which attains its maximum 
value at this temperature. 
On the first day, when we experimented with a solution 
which we had not previously used, the earlier and thinner 
showers of crystals did not appear, and the later and denser 
shower might be somewhat retarded. But in subsequent 
experiments the thin shower (accompanied by a slight diminu- 
tion in the rate of increase of the refractive index) occurred 
at exactly the saturation temperature, and the dense shower 
(accompanied by an actual diminution of the refractive index) 
occurred at a perfectly definite lower temperature. 
We infer that the first shower takes place because some fine 
crystalline particles of the dissolved substance get into the 
solution with the dust of the air, and continue to grow, and 
that the second shower is due to a different cause. 
If this be so, then the same solution enclosed in a sealed 
glass tube should not yield the first shower. 
We have made this experiment with numbers of aqueous 
solutions of different substances and of different strengths, and 
have always found that in a sealed tube the first shower is 
prevented, and that the liquid cannot be made to crystallise 
