to Calcium and Magnesium JMetasilicates. 47 



scope, fitted with electric resistance furnace, fig. 13a, lias been 

 constructed and found useful in the study of these etch pits. 



6. The specific-volume curve consists of three well-defined 

 branches, the first of which is the locus of the volumes of 

 mechanical mixtures of pseuclo.-wollastonite (a-CaSi0 3 ) and 

 diopside ; the second, that of the solid solutions of magnesian 

 pyroxene (/3-MgSi0 3 ) in diopside ; and the third the locus of 

 the volumes of mixtures of saturated mix-crystals just men- 

 tioned, and the free magnesian pyroxene. The volume of the 

 solid solutions is greater than the sum of the constituent 

 volumes. There is a sharp minimum on the curve at the 

 composition of diopside CaSi0 3 .MgSi0 3 . On account of the 

 presence of minute bubbles in the crystals and the compara- 

 tively small difference between the specific gravity of diopside 

 and that of the magnesian pyroxene, the critical points on the 

 curve are several per cent in error. 



7. A method for the more accurate determination of the 

 composition eutectics is described ; also a method for the 

 approximate location of inversion points in inert substances. 

 The accidental variations between different determinations of 

 the melting point of a sharp melting silicate seldom amount to 

 1° up to 1500°. This is the accuracy available for compara- 

 tive measurements. The absolute accuracy of a determina- 

 tion is less than this on account of the present limitations of 

 the absolute scale. 



The authors wish to express to Dr. Arthur L. Day their 

 hearty thanks for valuable assistance in connection with the 

 study of the a-magnesium silicate. 



Geophysical Laboratory, Carnegie Institution of Washington. 

 Washington, D. C, July 10, 1908. 



