Chemical Constitution of Sillimanite, 233 



a different view of its constitution. It is stated to be com- 

 posed of 



Silica, 38.670 



Alumina, 35.106 



Zirconia, 18.610 



Peroxide of Iron, 7.216 



99.602 



As it was obviously a matter of interest to endeavour to as- 

 certain which of the preceding analyses was the correct one, 

 I several times attempted to procure specimens of Sillimanite 

 from mineral dealers in Edinburgh and London, but without 

 success, owing to the rarity of the mineral in this country. I 

 have since, however, been indebted to the kindness of Mr Rose, 

 mineral-dealer, Edinburgh, for a small quantity of the mineral, 

 from the above-mentioned locality. The crystals were, as usual, 

 four- sided prisms, embedded in quartz, and possessing the or- 

 dinary external characters of the mineral. Colour varying from 

 clove-brown to yellowish-white. Cleavage perfect in one di- 

 rection, and shewing high lustre. Crystals bent ; some of them 

 translucent, others more opaque ; brittle, and readily pul- 

 verized. 



The quantity of the crystals which I was able to separate 

 from the matrix was considerably less than I could have wished 

 for the purpose of a regular analysis ; but the principal object 

 was to ascertain whether the mineral contained zirconia ; and 

 the quantity was quite sufficient to enable me to determine 

 whether it contained so large a proportion of that earth as 

 18.5 per cent. 



3.02 grains of the pure crystals were reduced to fine pow- 

 der, and moderately ignited for a quarter of an hour in a pla- 

 tinum crucible over a spirit-lamp, with rather more than three 

 times their weight of carbonate of soda. When cold, some- 

 what less than twice their weight of pure caustic potash was 

 laid on the surface of the powder, so as not to be in contact 

 with the crucible. The whole was then slowly heated to red- 

 ness over the spirit-lamp ; and the crucible afterwards trans- 

 ferred to a charcoal fire, where it was strongly ignited for 



