126 SCIENCE \PROGEESS 
low temperatures to trace this curve. But we may confidently 
predict that for an aqueous solution of a salt which separates in 
the anhydrous condition—or for a mixture of any two substances 
which do not form compounds or isomorphous mixtures—the 
usual V-shaped freezing-point curve of the two components 
(MEm of fig. 2) is accompanied by a V-shaped supersolubility 
curve (SHs of fig. 2) representing their temperatures of 
spontaneous crystallisation. 
[Since writing the above I hear from Mr. Hartley that he 
and Mr. Barrett have realised the ice supersolubility curve in 
studying the crystallisation of dilute solutions of sodium sulphite. | 
In general, therefore, the cooling of two substances dissolved 
in each other, whether an ordinary solution, a mixture of two 
fusible substances, a rock, or an alloy, will be represented by 
fig. 2, in which M and m are the melting points of the two sub- 
stances A and B, ME and mE their freezing-point curves meeting 
in the eutectic E; Sand o are the temperature of spontaneous 
crystallisation of A and B when molten and supercooled; SH 
and cH constitute their supersolubility curve, meeting in what 
we call the ‘‘ hypertectic” point H; this is the highest tempera- 
ture at which they can crystallise spontaneously together. 
The process of cooling of a liquid mixture initially of com- 
position X is generally supposed to be represented by the line 
xafE. The liquid cools down till it becomes saturated at a and 
then cools, with crystallisation of A, always remaining saturated 
(as represented by afE) until at E it reaches the eutectic com- 
position, when B and A can crystallise together without change 
of temperature, and the whole mass solidifies as the eutectic 
surrounding the crystals of A. 
It will now be clear that this only occurs if the liquid be 
inoculated with A, be kept stirred, and be cooled extremely 
slowly. Otherwise supersaturation will occur, and the process 
is, again, more faithfully represented by the line XabfE ; crystals 
may make their appearance by inoculation at the temperature a@ 
—but at the temperature 6 there will be a sudden access of 
crystallisation by reason of the spontaneous growth, and some 
time will elapse before the solution again falls to saturation 
point at f Or, before this happens, the solution may become 
labile for B, and that substance may crystallise spontaneously, 
and so the eutectic mixture may never be attained. 
In order to test these predictions we have made experiments 
