to Calcium and Magnesium Metasilicates. 7 



lowing a slightly curved line to the eutectic point, 1348°, at about 

 28 per cent magnesium silicate. The curve then rises much 

 more gradually to a maximum of 1380° at the composition of 

 diopside, CaSi0 3 .MgSiO s , which contains 46*3 per cent mag- 

 nesium silicate. Let us first consider so much of the curve by 

 itself. The solid phases, which separated when the mixtures 

 from per cent— 46*3 per cent magnesium silicate were crys- 

 tallized as nearly as possible under equilibrium conditions, were 

 proved by microscopic analysis to be only diopside and pseudo- 

 wollastonite, making of course the proper allowance for a small 

 mutual solubility. The latter amounted, as we shall see, to 

 about 3 per cent diopside in the pseudo-wollastonite, and less 

 than 3 per cent of the latter in diopside. 



The end members, pseudo-wollastonite and diopside, and also 

 the eutectic mixture, melt at a single temperature, with the 

 reservation just made ; that is, as sharply as we have yet ob- 

 served with any silicate. The other mixtures show two phe- 

 nomena : (1) the melting of whatever amount of eutectic is 

 present in the mixture ; and (2) as the temperature rises, the 

 gradual solution in the melted eutectic of the excess component 

 (diopside or pseudo-wollastonite, as the case may be), which lasts 

 till the melting point curve ABC is reached. The temperature- 

 time curves clearly show continuous change in character from 

 the end members to the eutectic composition. Near the end 

 member the predominant phenomenon is the upper point, 

 which resembles the sharp melting of the pure component 

 itself ; the eutectic melting is of course small. It is clearly 

 distinguishable, however, even when the mixture contains less 

 than 1 per cent of eutectic. In mixtures farther from the end 

 members in composition, the absorption of heat immediately 

 above the eutectic melting is perceptible, and the highest point 

 gradually takes on more and more the character of the end of this 

 absorption and less and less that of a separate and independent 

 melting point. Although, strictly speaking, this upper melting 

 is always a solution, the temperature at which it terminates did 

 not show, under the conditions described, any change with the 

 rate of heating. In order to test this question, the rate was in 

 several cases altered from 1° to 3° per minute, but the result- 

 ing effect on the upper point was less than the accidental errors 

 of the separate determinations. As the eutectic composition 

 is approached and the phenomenon has still more the character 

 of a solution and less that of a melting, the upper point 

 becomes less and less distinct. 



Determination of the Eutectic Composition. — In the imme- 

 diate neighborhood of the eutectic composition the difficulty of 

 determining the upper point is still more increased by another 

 phenomenon. The rapid temperature rise which immediately 



