THE PHYSICAL CHEMISTRY OF THE CRYSTALLIZATION 



AND MAGMATIC DIFFERENTIATION 



OF IGNEOUS ROCKS 



J. H. L. VOGT 



Trondhjem, Norway 



(Continued from page 649 of Vol. XXIX) 



V 



THE INFLUENCE OF PRESSURE 

 A 



The dependence of the melting point on uniform^ pressure is 

 designated, as is well known, by the formula established by Clausius- 

 Chapeyron : 



AT T(vKquid — VsoKd) 



AP = ^°-333 Ki:^ ^ 



AT . . 



-Tp designatmg the alteration of the melting point in C° per increase 



of atmospheric pressure, T the melting point in absolute tempera- 

 ture (starting from —273°), q the latent melting heat in gr. cal. pr. 

 gr., V the specific volume (in cm^ per gr.) at the melting point — 

 vuq. — Vsoi. accordingly meaning the difference of specific volume 

 between the liquid and the solid phase at the melting point— and 

 E = 425. 



In by far the most substances vnq.— Vsoi. is positive at common 

 and at moderate pressures. Accordingly the minerals, when 

 melting, must, as a general rule, expand in volume or become of 

 lower specific gravity. The case is entirely the reverse of that of 

 ice and of bismuth. 



^ Within a liquid, such as a magma, also in the case that more or less mineral has 

 already crystallized, the pressure always becomes imiform or hydrostatic, and in this 

 case the formula quoted is applicable. Quite different is the case of non-uniform com- 

 pression (and stress) in the solid phase. I beg to refer to a series of publications of 

 recent years (see the review of John Johnston in this Journal, XXIII [1915], p. 732). 



611 



