386 REPORT—1905. 
Vogt even goes a step further in his application of the principles of modern 
chemistry. The order of crystallisation appears to be by no means always that 
of the solubility, but indicates that a mineral is sometimes not so soluble as might 
be supposed. Now another principle in the modern physics of solutions is that 
by adding to a solution of one substance a new electrolyte containing an ion 
common to both the solubility of the first is diminished, and Voet does not 
hesitate to apply this principle. 
Thus spinel and felspar in mutual solution, when felspar is in large excess, 
should on cooling yield felspar first. But in many basic rocks spinel is the first 
to crystallise; this is, according to Vogt, due to the presence of ferro-magnesian 
silicates containing the Mg-ion which is also present in spinel ; if these be partially 
dissociated the solubility of the aluminate will be lowered. 
An obvious criticism on this argument is that if the dissociation is so slight 
that it may be ignored for one purpose, it is hardly fair to invoke its powerful 
action for another, and it is possible that Vogt in his enthusiasm for a theory 
attempts to explain too much by its aid. 
It is clear, however, that the labours of Vogt have been precisely in the 
direction indicated by Teall in the words that I have quoted, ‘experiment con- 
trolled by the modern theory of solution’; and if his opponents are tempted to 
think that he may have carried the principle too far with insufficient data, they 
cannot but admire the brilliancy, the persistency, and the ingenuity with which 
he has applied the newer theories of solution at every turn. 
Heycock and Neville’s Work on Alloys. 
I must next refer briefly to another remarkable series of researches which have 
recently been published. 
The laws which govern the solutions of metals in metals, that is to say alloys, 
appear to be the same as those which prevail in the case of other solutions; it is 
in alloys that the nature of eutectic mixtures has been most fully studied ; and the 
phase-rule and Roozeboom’s deductions from it have been applied with signal 
success to their investigation, A new impulse has been given to the subject by 
the work of Heycock and Neville which is summarised in their Bakerian lecture 
delivered last year upon the copper-tin series of alloys. They have studied the 
changes which occur during the cooling of an alloy by taking small ingots of the 
cooling metal and chilling them at certain temperatures; this arrests the gradual 
process of cooling and causes all that is liquid at the moment of chilling to become 
suddenly solid ; it is then possible by polishing and etching the ingot to sbow the 
solid crystals set in the congealed ground-mass and to study ther nature. They 
have been able to interpret their results by means of Roozeboom’s remarkable work 
on the solidification of mixed crystals published in 1899. For our present purpose 
it is suflicient to consider these results as applied only to alloys. Ifa diagram be 
constructed with the temperatures for ordinates and constitution for abscisse, 
Roozeboom has shown that two curves may be drawn. The first is the freezing- 
point curve, or /iguidus, giving the temperatures at which an alloy of any compo- 
sition begins to solidify : this is a broken curve and each section of it represents 
the temperature of equilibrium between the liquid and a different solid alloy ; the 
breaks represent the temperatures and constitution of the liquid at which one 
solid ceases to he produced and another begins. The curve is, of course, far more 
complicated than the simple Y of Meyerhotfer, since that represents the cooling of 
a mixture whose constituents do not form compounds or isomorphous mixtures, 
whereas the alloys do both. In this respect the alloys resemble a silicate magma 
which is crystallising as a rock-mass; indeed it will be remembered that Mende- 
léef insists upon the general similarity of silicon compounds to metallic alloys. 
The second curve of Roozeboom is the melting-point curve, or solidus, repre- 
senting the temperatures at which an alloy of given composition becomes com- 
pletely solid. Points above the liquidus represent the condition of alloys which are 
completely liquid ; points below the solidus that of alloys which are completely 
solid ; points between the two that of cooling alloys which are only partially solid ; 
