1 64 SCIENCE PROGRESS. 



"Soret's principle". In this view a molten rock-magma 

 is regarded as of the nature of a solution, the constituent 

 which is on the point of crystallising out at any stage of the 

 consolidation being considered as dissolved in the residual 

 liquid, or some special silicate-compound being regarded 

 as the solvent. Now Van 't Hoffs theory of osmotic 

 pressure teaches that if different parts of an ordinary 

 saline solution be at different temperatures, the concen- 

 tration will also vary, equilibrium being attained only 

 when the concentration in different parts is inversely 

 as the absolute temperature. Experiments tending to the 

 same general conclusion had previously been made by 

 Soret. This idea, applied to rock-magmas, has been held 

 to afford some explanation of the relative concentration of 

 the basic constituents in the cooler part of a magma-basin. 

 These constituents, which are the first to separate out when 

 crystallisation begins, are regarded as the least soluble. 



The applicability of Soret's principle, at least in its 

 simple form, to the case of rock-magmas has been doubted 

 in various quarters. It is easy to see that the case of a 

 silicate-magma is much more complex than that of a single 

 salt dissolved in water, and in dilute solution. Backstrom/ 2 

 in particular, has adversely criticised the theory, and prefers 

 to the analogy of a saline solution that of a mixture of 

 liquids. Taking an example, he remarks that water and 

 aniline, if mixed at ordinary temperatures, separate into 

 two layers, one consisting of water with about one per cent, 

 of aniline, the other of aniline with about two per cent, of 

 water. If the temperature be raised, the proportions of 

 aniline in the former layer and water in the latter increase, 

 until at 1 66° the distinction between the two layers dis- 

 appears. This property of mixing freely above a certain 

 temperature, and separating more and more completely as 

 the temperature falls below that degree, is, he maintains, a 

 property common to all liquid-mixtures where no chemical 

 action takes place ; and he proposes to apply it to the case 

 of fluid rock-magmas regarded as mixed liquids. 



