476 Rejyorts and Proceedings — British Association. 



the constituents, as calculated by Van 'fc Hoflf'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 Si Og 

 or SisOg. 



Vogt boldly attacks the whole problem as one that can be solved 

 on these lines : we have good reason to believe that the slags and 

 rock-magmas are solutions ; we know their constituents ; we can 

 therefore proceed to experiment with these constituents and to 

 predict the behaviour of their mixture according to the principles of 

 physical chemistry. The order of ci'ystallisation is mainly deter- 

 mined 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, 

 dioj^side, akermanite, melilite, and anorthite have the simplest 

 possible formulfe corresponding to their analyses, and are not 

 polymerised. Mineralogists will welcome this conclusion if it be 

 true, for it has occasionally been the fashion on theoretical grounds 

 to attribute a high degree of polymerisation to many minerals, and 

 nothing is easier than to account for many difiSculties if one may 

 multiply the formula of a mineral by any number that is required. 

 It should be added, howevei", that Doelter, calculating from his own 

 experiments, is led to think that some of the minerals must have 

 formulge which are multiples of their empirical formulas. 



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 Yogt 

 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 dis- 

 sociated the solubility of the aluminate will be lowered. 



An obvious criticism on this argument is that if the dissociation is 

 60 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 controlled 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. 



