3o6 EDSON S. BASTIN 



of magmas and to produce large textural variations. In this connec- 

 tion it may not be out of place to call attention to a possible applica- 

 tion of Raoult's Law.^ This law states that if various substances are 

 dissolved in equal amounts of the same solvent in the proportions of 

 their molecular weights the resulting lowering of the freezing-point 

 of the solution will be the same in each case.^ In other words the 

 effect produced is a function of the number of molecules concerned 

 and is not primarily dependent on the nature of the substances intro- 

 duced. It follows that a small amount by weight of a substance of 

 low molecular weight (such as HjO, mol. wt. i8) will exert the same 

 depressing influence on the freezing-point of the solution as a much 

 greater weight of a substance of high molecular weight (such as Fe203, 

 mol. wt. 1 60), and that given equal weights of the two, the substance 

 of low molecular weight will exercise much the greater influence. This 

 law has been found to apply strictly only to very dilute solutions where 

 there is no chemical action between solvent and dissolved substance. 

 It has been applied bodily by Vogt'^ to rock magmas, but the wisdom 

 of such extension to cover widely different and much more complex 

 physical conditions may well be questioned. It seems not unreason- 

 able, however, to attribute some general importance to this principle 

 in rock magmas, to the extent that magmatic constituents of low molec- 

 ular weight may exert greater influence in lowering the freezing-point, 

 decreasing viscosity, and affecting textures than constituents of high 

 molecular weight. They may thus attain an importance which 

 appears disproportionate to the small part by weight which they form 

 of the whole magma. The substances (hydrogen, water, fluorine, 

 chlorine, and boron) commonly believed to exert the greatest influence 

 upon the viscosity of magmas and the textures of the resulting rocks 

 are all substances of much lower molecular w^eights than silica and 

 the rock-making silicates and oxides, even when the minimum values 

 for the latter are assumed. The hiatus between the molecular weights 

 of these two groups of substances is so marked as to justify the reten- 

 tion of the term " mineralizers " for the lighter group, in case the 



1 See Ostvvald, Outlines of General Chemistry, 136-37 (1895). 



2 Neglecting electrolytic dissociation, which is probably of small importance in 

 rock magmas. 



3 See Vogt, Die Silikatschmelzlosungen, II, 128-35 (i9°4)- 



