Day and Shepherd — Lime-Silica Series of Minerals. 281 



three polymorphic forms in enantiotropic relation to each 

 other, which we have designated as a, j3 and 7, in the order in 

 which they form from fusion. The a-form is the only modi- 

 fication which is stable in contact with the melt. Its specific 

 gravity is about 3 '27, determined in methylene iodide solution 

 upon fresh crystals. Its hardness is 5-6, Mohs's scale ; crystal 

 system, monoclinic. 



Below 14:10° the a-form changes into the /3-form, of which 

 the density 3*28 (measured by comparing the indices of refrac- 

 tion) is but little different from that of the a modification. 

 The substance was too unstable for determinations of the 

 density to be made in the ordinary way. It crystallizes in the 

 orthorhombic system. The inversion point between the a and 

 /3 varieties is well marked and distinguishable over almost the 

 entire range of compositions of which the orthosilicate is a 

 component, as indicated in the diagram (fig. 3, line MN). The 

 inversion of (3 into 7 occurs at about 675° with a large increase 

 of volume which at once explains the disintegration of the 

 material. The temperature at which this inversion occurs is 

 somewhat variable, and it is ?tot readily reversed. It is much 

 too slow a change to admit of pyrometric determination, but 

 it is possible to locate it approximately by quenching the 

 material from selected temperatures in the neighborhood of 

 the inversion point. The usual procedure was to take a small 

 portion of the disintegrated material, fold it tightly between 

 thin strips of platinum and place these in the furnace. The 

 temperature was then raised to any chosen value and main- 

 tained constant for periods of time varying from six hours to 

 three or four days. At temperatures far enough removed 

 from the inversion point, the transition from one form into 

 the other was fairly rapid, but as the temperature of inversion 

 approaches, equilibrium is attained with increasing difficulty. 

 After the furnace had remained at this constant temperature 

 for a length of time, it was opened, the platinum strips contain- 

 ing the orthosilicate were removed and quickly plunged into 

 mercury. In this way, from temperatures just above the 

 inversion point, it was possible to fix the /3-form long enough 

 to allow of its optical determination. 



The disintegration on cooling appeared to depend consider- 

 ably upon whether or not the a-form had first been allowed 

 to change into the /3-form. For example, if a small portion 

 of the orthosilicate is fused before the oxyhydrogen blast and 

 then plunged directly from the flame into mercury, the 

 quenched material will usually be stable for a considerable 

 time. If the flame is removed but a moment and the slightly 



Am. Jour. Sci.— Fourth Series, Vol. XXII, No. 130.— October, 1906. 

 20 



