Day and Shepherd — Lime-Silica Series of Minerals. 289 



pure components together with the eutectics which they form. 

 It only remains to study their thermal properties somewhat 

 more consistently with the help of the apparatus which has 

 been described. It was not found possible to determine the 

 presence of the eutectic (Line HBI, fig. 3) in the 10 and 20 

 per cent compositions, for reasons which will have become 

 sufficiently clear already, but the microscopic evidence shows 

 the tridymite to be normal, whether it forms from pure silica 

 or in the presence of lime, so that the eutectic must extend 

 over to the silica axis. From 30 per cent on there was no dif- 

 ficulty in observing it pyrometrically. The observations are 

 included in Table I. 







Table I. 











Eutectic. 



Tridymite + Pseudo 



■wollastonite . 









(Line H I, fig. 30 









J erc 



entage of CaO. 



30 



32 



35 



40 



45 



iut 



ectic melts 



1420° 



1417° 



1120° 



1421° 



1419° 









1419 



1423 

 1414 



1420 

 1422 



1419 

 1406 





Mean 







1416 



1420 



1406 





1420° 



1418° 



1418° 



1421° 



1413° 



The liquidus AB (fig. 3) has been drawn as a dotted line. 

 The value assumed for the melting temperature of silica is 

 based upon considerations which have been elaborated elsewhere 

 (p. 272). It requires no further comment except perhaps to 

 call attention to the fact that it is much lower than the tem- 

 perature usually assigned to it. As the mixtures grow richer 

 in lime, the melting of the excess of silica seems to be consid- 

 erably facilitated, but there are no points sufficiently sharp to 

 serve any purpose as quantitative determinations until we 

 reach the composition 30CaO, T0SiO 2 . The microscopic evi- 

 dence is however both satisfactory and sufficient as to the nature 

 and continuity of the curve. 



Along the branch BC (fig. 3) of the liquidus, the pseudo- 

 wollastonite is the solid phase. It crystallizes from these mix : 

 tures in laths between which an extremely fine-grained, almost 

 sub-microscopic eutectic appears. 



It may be remarked in passing that the "eutectic structure" 

 in minerals is by no means so characteristic as in the case of the 

 alloys. Owing to the great viscosity of these melts and conse- 

 quent slowness of diffusion, it is evident that there is no oppor- 

 tunity for the formation of the characteristic grain structure 

 which we have" come to associate with the eutectics of alloys. 

 This almost complete absence of diffusion in silicate melts makes 

 it necessary to proceed with great caution in applying to minerals 

 the methods which are easily and effectively applied to the alloys. 

 For example, in alloys it is possible to crystallize out a portion 



