to Calcium and Magnesium Metasilicates. 15 



Solid solution determined by microscopic homogeneity ex- 

 tends as far as 66'5 per cent MgSiO,: 33*5 per cent CaSi0 3 . 

 The solid phases separating on the branch of the curve DE 

 were found to be the mix-crystal just mentioned, and 

 «-MgSi0 3 in all cases. All the mixtures from about 68-95 

 per cent MgSi0 3 showed an absorption of heat at 1375°. This 

 is evidently a eutectic line and there is no doubt that we have 

 Jhere a case of Roozeboom's mix-crystal Type V,* where each 

 of two mix-crystals lowers the melting point of the other to a 

 eutectic. Points on this line to the left of the eutectic point 

 could not be located since the liquidus curve lies only 2° to 3° 

 higher up. The eutectic composition is about 68 per cent 

 MgSi0 3 : 32 per cent CaSi0 3 and consists therefore of about 



32* 



_ — = 95*5 per cent of the diopside mix-crystal and about 4*5 



Oo'D . 



per cent of free a-MgSi0 3 ( in which about 2 per cent CaSi0 3 

 or 3*7 per cent of diopside is dissolved ). 



Inversion of MgSiO y — About 1365° pure magnesium silicate 

 undergoes a reversible change into an orthorhombic form. 

 The heat of transformation is small, and apparently the change 

 does not all happen at once, but extends over quite an inter- 

 val of temperature. This is the only satisfactory way of 

 accounting for the fact that while a small absorption of heat 

 is observed in most of the mixtures of magnesium silicate from 

 70 per cent on, in the pure substance and in the mixtures near 

 100 per cent MgSi0 3 it was entirety overlooked. The exist- 

 ence of a form stable above 1365° was proved by crystallizing 

 a melt near the melting point, and then suddenly chilling the 

 crystals in water. Thus the form was instantly brought into 

 a temperature region where viscosity was great enough to 

 prevent an inversion. The inversion temperature was deter- 

 mined approximately as follows : One tenth of a gram of the 

 substance was placed in a small platinum tray shaped from a 

 piece of platinum foil. This was suspended by a fine plati- 

 num wire inside the furnace. After the material was melted 

 and crystallized near the melting point, the temperature was 

 lowered to a given temperature and held there about a half 

 hour. The furnace in which the heating was done was 

 designed for calorimetric purposes, and had a removable 

 bottom which was swung aside at the proper moment, 

 when the suspension wire was instantly melted by an electric 

 current, allowing the charge to drop into a basin of water. 

 The results were as follows: 



*Zeitschr. f. Phys.-Chem., xxx, 403. 



