TRANSACTIONS OF SEOTION C. 385 
oxygen ratio or acidity, the various minerals of which they consist make their 
appearance within fairly well-defined limits, For example, in the case of the 
Ca-Mg-Fe-Mn slags, which contain little alumina, olivine and the melilite 
minerals only make their appearance in the more basic slags, and the meta- 
silicates in the more acid, the limit between the two corresponding to an acidity 
of about 1:6. 
The limit of individualisation between the various minerals is supposed to 
correspond to their eutectic mixture. Such slags may, therefore, be regarded as 
a mutual solution of two or more of the minerals olivine, enstatite, hypersthene, 
augite, the gehlenite-melilite group, akermanite, wollastonite, and the hexagonal 
metasilicate, which is so characteristic of the more acid slags. The particular 
minerals which make their appearance are practically determined by the acidity 
of the magma and hy the relative proportion of the bases present, particularly by 
the ratio of the calcium to the magnesium-iron-manganese group; in other words 
Vogt asserts that a silicate magma is a mutual solution of the various crystalline 
compounds that actually make their appearance as it solidifies, and that the order 
of crystallisation depends upon their proportion in the magma as compared with 
their proportion in the eutectic. The old conception of a solvent and a solute 
ceases to have much meaning; the matter which is of supreme importance is the 
nature oi the eutectic mixture when the constituents are given; thus micro- 
pegmatite and microfelsite represent the eutectic of felspar and quartz, and 
correspond to a mixture of about 74 parts of felspar to 26 of quartz, as indeed has 
been stated by Teall. 
Now, if we are justified in regarding rock-magmas and fused silicates as 
mutual solutions of certaia definite compounds, and if these compounds are actual 
minerals or other silicates which crystallise out of the magma when it cools, we 
are also justified in making use of the properties of these minerals when we apply 
to the magma the known physico-chemical laws which govern solutions. 
The number and nature of the minerals which can be in equilibrium with 
each other and the solution are to be determined by experiments upon their 
solubility interpreted by the phase-rule of Willard Gibbs, and especially by the 
laws which Roozeboom and other physical chemists have deduced for components 
which form double salts or isomorphous mixtures. Knowing the components we 
ought, therefore, to be able to determine their latent heat of fusion, their specific 
heat, the lowering of the freezing-point of their mixtures, and from these data 
to calculate the true formule of the rock-forming minerals, It will readily be 
understood that in a mixture of quartz and orthoclase, the lowering of the 
freezing-point below that of either of the constituents, as calculated by van 
*t Hoft’s formula, from their melting-points, latent heats, and molecular weights, 
will be very different according as the formula of quartz is taken to be SiO, 
or 5i,0,. 
Vogt boldly attacks the whole problem as one that can be solved on thesa 
lines: we have good reason to believe that the slags and rock-magmas are solu- 
tions; we know their constituents; we can therefore proceed to experiment with 
these constituents and to predict the behaviour of their mixture according to tha 
principles of physical chemistry. The order of crystallisation is mainly determined 
by the relative composition of the magma and the eutectic, and the composition 
of the eutectic may be calculated from the intersection of the freezing-curves. 
One interesting result is the conclusion that in the silicate magmas which 
have been the subject of experiment the minerals produced are all of very simple 
constitution ; that, for example, olivine, diopside, akermanite, melilite, and 
anorthite have the simplest possible formule corresponding to their analyses 
and are not polymerised. Mineralogists will welcome this conclusion if it ba 
true, for it has occasionally been the fasbion on theoretical grounds to attribute 
a high degree of polymerisation to many minerals, and nothing is easier than ta 
account for many difficulties if one may multiply the formula of a mineral by 
any number that is required. It should be added, however, that Doelter, calen- 
lating from his own experiments, is led to think that some of the minerals must. 
have formule which are multiples of their empirical formule. 
1905. co 
