Febbuaet 5, 1909] 



SCIENCE 



211 



able extent. Apparently liquid silicates 

 are miseible in one another in all propor- 

 tions, though suggestions that they may 

 not be have been advanced by some petrol- 

 ogists. It is known that liquid sulphides 

 and silicates are not miseible in all pro- 

 portions at all temperatures. And where 

 sulphides exist in large amounts, separa- 

 tion in the liquid phase may take place 

 with falling temperature. 



Separation of solids from solution de- 

 pends upon the attainment of a sufficient 

 molecular concentration of substances to 

 saturate the solution. Saturation may be 

 brought about in several ways: by chemi- 

 cal reaction within the solution consequent 

 upon a change of chemical equilibrium ; by 

 change of temperature, usually by lower- 

 ing temperature; by change of pressure, 

 either acting in an opposite maxiner from 

 temperature or by affecting the gas con- 

 tent. 



Solids may separate when the point of 

 saturation for them has been reached, or 

 the liquid may become superheated, and 

 separation be delayed. In this condition 

 separation is often induced by the inser- 

 tion of a solid of like composition, or of an 

 isomorphous compound, or by agitating. 

 Such a condition of a liquid has been 

 called metastahle, and in this condition, 

 as shown by Miers in laboratory observa- 

 tions on liquids of organic compounds, 

 crystallization of the separating substance 

 takes place, at relatively few points, and 

 proceeds gradually, according to degree of 

 concentration and other factors, until com- 

 paratively large individuals are formed. 

 If supersaturation proceeds without sepa- 

 ration of solid phase a point will be 

 reached when separation will take place 

 spontaneously at many points in the liquid 

 and continue rapidly. This is the labile 

 condition of the liquid. When this con- 

 dition is reached by a cooling liquid crys- 

 tallization often takes place suddenly as a 



shower of minute individuals, as observed 

 by Miers. The bearing of these facts on 

 the textures of igneous rocks is apparent, 

 and a knowledge of the laws relating to 

 the separation of solids from liquids; the 

 order in which those of different sub- 

 stances may follow one another in a mixed 

 solution; the separation of isomorphous 

 compounds; and the shapes that may be 

 assumed by the resulting crystals of vari- 

 ous minerals lead to an understanding of 

 the texture of igneous rocks. 



A supersaturated condition is more 

 readily obtained in more viscous liquids, 

 which are more apt to solidify without 

 crystallization, as glasses, than more fluid 

 liquids. The most familiar illustrations 

 of this law among igneous rocks are the 

 persilicic (rhyolitie) lavas, which often 

 form glasses (obsidians). The question 

 has been raised by Crosby, and others, 

 whether an earthquake happening when a 

 magma was in a sufficiently supersatu- 

 rated, metastahle, condition might not in- 

 duce crystallization of some of the con- 

 stituent compounds. 



Crystallization may begin with differ- 

 ent degrees of supersaturation of the 

 liquid, and would proceed at different 

 rates according to the degree of supersatu- 

 ration, being more rapid the greater the 

 concentration. It would also be more rapid 

 the greater the molecular diffusivity, that 

 is, the lower the viscosity of the liquid, 

 and the greater the rate of cooling, so long 

 as this does not increase viscosity too 

 rapidly. Gradual or slow crystallization 

 at comparatively few centers would yield 

 relatively few, large, crystals; whereas 

 sudden, rapid crystallization from many 

 centers would produce many small ones. 



High fluidity in solutions would permit 

 easy diffusion of separating molecules 

 toward crystallizing centers, favoring the 

 growth of relatively large individuals. 

 High viscosity would retard diffusion and 



