TRUST WORTPIINESS OF MELTING TEMPERATURES 151 



which it appeared that the order of crystallization as seen under the 

 microscope was not always the same, even though the chemical composi- 

 tion was approximately so. It is also possible for the first crystals which 

 separated to redissolve at lower temperatures, so that the cold rock does 

 not necessarily contain in itself a complete history of its crystallization. 

 Such physical conditions as rate of cooling and the presence or absence 

 of nuclei about which crystals might begin to form enter into this prob- 

 lem as determining factors. It is probably unwise, at this early stage 

 in the development of the subject, to draw sweeping conclusions from 

 these relations, except, perhaps, to emphasize the fact that the order of 

 crystallization or differentiation from the magma will hardly be deter- 

 mined on the basis of chemical composition alone, though of course it 

 must be fixed by the law of minimum potential. 



Can TRUSTWORTHY Melting Temperatures be obtained? 



It remains for us to pursue the question on its constructive side, and 

 to inquire whether trustworthy melting temperatures can be determined 

 for the minerals in the face of the limitations which many of them have 

 been seen to possess. Obviously not by the simple and heretofore gener- 

 ally acceptable process of using the sanie subjective criterion of the mo- 

 ment of melting for all minerals alike ; namely, the change in the appear- 

 ance of the mineral in a crucible during heating. Between the latitude 

 which can be allowed to the judgment of the observer and the wide differ- 

 ence in the properties of the different minerals themselves, this method 

 will fail, as it has done heretofore, to yield uniform and therefore trust- 

 worthy results on which to base serious geological conclusions. On the 

 other hand, if we choose for each mineral an appropriate property which 

 shows a conspicuous change corresponding to the change of state, or, still 

 better, if we leave the personal judgment of the observer out of the ques- 

 tion entirely, at least wherever it can be done, and measure by a sensitive 

 method the change in the energy content of the system which is invariably 

 coincident with its change of state, whatever the external evidence of the 

 latter may be, we shall arrive, with nearly all the minerals, at perfectly 

 definite constants, which may be redetermined at will, and which always 

 possess physical significance. 



Assuming that we have reached this standpoint, that the individual 

 limitations in the properties of the minerals must be respected and that 

 the judgment of the observer must be confined to conspicuous properties, 

 or, still better, wholly eliminated, the analysis of melting-point deter- 



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