128 SCIENCE PROGRESS 
nitrate, 7e. for mixtures of three components, the two salts 
and water. 
No doubt the same principles prevail in the solidification of 
metals, alloys, and rocks, although the problem is here compli- 
cated by the formation of compounds and isomorphous mixtures 
(solid solutions); but whatever substance solidifies, it can only 
crystallise from a solution which is supersaturated with regard 
to it, and unless this is made to take place by inoculation, 
the solution must first become sufficiently supersaturated to 
crystallise spontaneously. 
The freezing-point curves terminate at M and m in the 
melting points of the pure substances A and B; and the super- 
solubility curves terminate at S and s, the temperatures of 
spontaneous crystallisation of A and B. We have determined 
these for salol and betol. We have also made experiments with 
water, and have found that pure water contained in sealed tubes 
crystallises by shaking at —1’9° C. ; and even if heavy substances 
be enclosed with the water in the tubes so as to produce 
friction, we have been unable to make it crystallise at a higher 
temperature than —o'4° C. 
If the crystallisation of a drop of any solution be watched 
under the microscope, remarkable changes in the process will 
generally be noticed so soon as the drop passes from the 
metastable to the labile condition. Experiments made in my 
laboratory by M. Chevalier have shown that, in a drop of labile 
solution, potash-alum crystallises as a rectangular network of 
fine needles, and not in the ordinary octahedra. This observa- 
tion accords with the general experience that when crystals 
grow very rapidly they usually grow as needles. I believe this 
to be due to the fact that when a crystal has started in a labile 
solution the liquid in immediate contact with it is at once 
reduced to the metastable condition both by loss of solute and 
by rise of temperature, so that the growth only takes place 
within the sheath of less concentrated liquid with which the 
crystal envelopes and protects itself, and therefore proceeds 
quite slowly. But if any point of the crystal has begun to 
advance so rapidly that it projects through this sheath before 
the zone of weaker solution has time to close round it, then 
it may continue to advance rapidly through the strongly super- 
saturated labile solution ; and the concentration currents which 
play most energetically upon the end of a crystal will ensure 
