INCOMPLETE REACTIONS 175 



a cooling solution, and may take a stable or an unstable form, the unstable 

 form will appear first,^'^ and may go over into the stable form afterward, 

 or may not, according as favorable conditions happen to occur or not. 

 Enstatite is a familiar case of an unstable mineral which has probably 

 never attained its stable form except in the laboratory. 



The Application of Van^t Hoff's Law — Vogt 



The most comprehensive attempt to apply these principles of solutions 

 to the crystallization of igneous rock magmas is unquestionably that of 

 Professor Vogt, of Christiania. He employed for the purpose a series of 

 artificial slags out of which he was able to crystallize most of the ordinary 

 constituents of the basic igneous rocks, and the results of his experiments 

 are in reasonably close analogy to the crystallization of these rocks except 

 for the volatile constituents, and the absence of these is probably a suffi- 

 cient explanation of the absence of those minerals which are missed from 

 his series, such, for example, as the alkali feldspars, hornblende, musco- 

 vite, and probably quartz. Yogf s investigations were along two principal 

 lines. He sought to ascertain what mineral first crystallized from a solu- 

 tion of given composition, and, second, whether the lowering of the melt- 

 ing point through the addition of one component to another in such a 

 magma follows the Yan't Hoff-Eaoult law.^^ In pursuing the first in- 

 quir}^, he was able to establish for most cases that the first mineral to sepa- 

 rate is the mineral in excess of the eutectic proportion. This is the more 

 noteworthy in view of the fact that Vogt made no special effort to obtain 

 or study pure minerals, and his results are therefore wholly qualitative in 

 character. The second investigation, Part II of Vogt's principal memoir, 

 did not result in equally definite conclusions. He was able to establish 

 the fact that the addition of one mineral to another or to a definite mix- 

 ture of minerals, in general lowers the melting point, as we have just ex- 

 plained, but he was not able to offer satisfactory quantitative evidence of 

 the amount of this lowering. This may have been due to two difficulties : 



23 The demonstration Is given in Ostwald, loc. cit. 



28 J. H. Van't Hoff : Die Rolle des osmotischen Druckes in der Analogie zwisctien 

 Losungen und Gasen. Zeitschr. f. phys. Chiera., 1, 1887, pp. 481 — . 



.T. H. L. Vogt, Der Sililvatschmelzlosungen, II, 1904, p. 128. Vogt applies the law 

 In this form : 



m .02 Ta 



M q 



T= absolute temperature of the melting. 



t = amount of lowering of melting temperature. 



q = latent heat of fusion per gram of solvent. 

 M = molecular weight of the dissolved substance, 

 m = concentration. 



I 



