878 



lions are ranged in tlie following, in series of increasing (inantities 

 of SiO,. 



a, 91.8 Weight Proc. SiO, (= 84.7 Mol. Proc). 



This preparation showed two components: tirstly a snbstance of 

 weak birefringence and a refractive index of about 1.480. It is 

 present in irregular felty needles, spread through the other component, 

 or in corns of indefinite boundaries. This substance seems to be no 

 other than «-trid3inite. 



Another snbstance of stronger birefringence and of a positive 

 character, with a refractive index of about 1.545 is seen accompa- 

 nying it. The birefringence is about : 0.020, thu^ being a little less than 

 for the metasilicate. This com[)oncnt is the bisilicate: Li^Si.^O^. Solid 

 solutions between this compound and SiO,, which we were inclined 

 to suppose, seem after Dr. Wkight's jndgment not to be present. 



h. 89''/o SiO, (= 80 Mol. Proc). This preparation is in all respects 

 analogous to the former one. 



Kig. K». 89 Weighl rruc. SiO. 

 (Enlaigement : 50 X'- 



Fio'. 11. 8:.,2\, SiU... (Eiilaigement: 50 X)- 



r. 85.3» „ SiO, (=74.2 Mol. Proc). 



The cpiantitj of tridymite is considerably diminished; it is only 

 present in exceedingly small needles. The bisilicate presents itself in 

 tabular crystals, intergrow^i with tridymite. In convergent polarised 

 light, the substance is shown to be biaxial; however the axial angle 

 seems to be too large, to observe a complete interference-image. 

 Sometimes the crystals have the aspect as if subjected to torsion. 



d. 82.17o SiO^ {= 69.4 Mol. Proc). This preparation is almost homo- 

 geneous, and consists of tabular crystals; the macroscopical aspect 



