TRANSACTIONS OF SECTION C. 387 
and the curves themselves show which solid compounds can be in equilibrium with 
the liquid and with each other at any temperature. 
The cooling-curves of Roberts-Austen and Stansfield had shown that con- 
siderable evolutions of heat may occur in cooling alloys far below the temperature 
of solidification, indicating that changes are going on in the solid as well as in 
the Jiquid condition. Heycock and Neville carry their investigations below the 
temperature of complete solidification and study these changes also. 
In the case of the copper-tin series of alloys they find that, according to the 
temperature and constitution of the liquid, crystals belonging to no less than six 
different types may separate, namely :— 
a, a solid solution of Cu with less than 9 per cent. of Sn. 
8, a solid solution of Cu with less than 27 per cent. of Sn. 
y, of which the constitution is not known. 
5, which probably has the composition Ou,Sn. 
7, Which probably has the composition Cu,Sn. 
H, which probably has the composition CuSn. 
Both £ and y are unstable at ordinary temperatures. The compound 6 crystallises 
out of 8 or y while they are already in the solid state, when the temperature falls 
sufficiently. 
A glance through the 101 photographs of chilled and etched ingots which accom- 
pany Heycock and Neville’s paper on this series of alloys shows how impossible it 
would be from the final composition of the solid alloy to ascertain the various 
stages through which it has passed: during cooling; as the authors remark, it is of 
the nature of a palimpsest. For example, the alloy, containing 14 atoms of tin to 
86 of copper, consists at 800° of a crystals in a ground-mass which probably con- 
tains £; it solidifies at about 775° ; at 675° there are only 8 crystals; at 600° there 
are a and £ crystals, but here a has crystallised out of 8 after it became solid ; at 
530° there is a much larger proportion of a; at 470° there are a crystals immersed 
in a mixture of a and 6 into which the residual 8 has broken up on cooling. 
If the course of events is so complex in an alloy of ouly two metals, how much 
more difficult must it be to decipher in the case of a mass of complicated silicates 
which are even more prone to form isomorphous mixtures, such as we have in a 
solid yock, not to mention the additional presence of aluminates, oxides, and 
sulphides. And yet geologists are accustomed to speculate freely about the crys- 
tallisation of rock constituents from the magma without taking account of any- 
thing save the final stage. 
I cannot help thinking that the experimental method of Heycock and Neville 
will have to be applied to the study of slags and fused silicates if we are to trace 
successfully the evolution of rock species. The value of their work to geologists 
is not only that the results are skilfully interpreted by the light of modern physical 
chemistry, but primarily that it is experimental work upon actual crystallising 
materials. 
Supersaturated Solutions. 
I do not myself see how we can do otherwise than apply to the study of rock- 
magmas all that can be learnt from physical chemists concerning the behaviour ot 
solutions, for though we cannot attain in laboratory experiments the high tempera- 
tures and great pressures at which rocks may have crystallised, there is no reason 
to believe that these introduce more than a difference of degree. The principles ot 
equilibrium between the various crystallising components probably remain the 
same, whatever may be the temperatures and pressures at which they have 
soliditied. 
It must at the same time be confessed that most of the experiments upon which 
the modern theory of solutions has been built up have been conducted upon dilute 
solutions, whereas the problems of crystalline growth are concerned, not with dilute 
nor even with saturated solutions, but only with solutions which are supersatu- 
rated. There is some force in the objection of Doelter tiat the results of such 
experiments may not be directly applicable to crystallising slags, 
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