1820.] Dr. Thomson's Analysis of Needlestone. 40& 



mesotype must be discarded altogether. This, however, Heave 

 to Mr. Brooke's discretion. 



The specific gravity at 60° I find 2-36966. This corresponds 

 perfectly with 2-37, assigned by Mr. Brooke. Indeed, as the 

 specimen^ were pure, and from the same place, any difterence 

 between our determinations could only have been ascribed to 

 error on one side or the other. 



It is liard enough to scratch fluor spar and stilbite, but is- 

 itself scratched by felspar. It is brittle. Easily frangible. The 

 fragments columnar, and the colour snow-white. 



Before the blow-pipe, it swells up like borax, and becomes 

 opaque, and snow-white, but does not melt. When exposed to- 

 a red heat, it becomes opaque, very white, and shining, similar 

 to enamel, or the finest porcelain. The edges are rounded, but 

 it does not altogether lose its shape. By this treatment, it 

 loses 13 per cent, of its weight, and its specific gravity becomes 

 2-7166. 



I shall now relate the experiments Avhich I made to determine 

 the constituents of this mineral. 



Fifty grains of it previously reduced to a fine powder were 

 digested for 24 hours in nitric acid upon the sand-bath. The 

 powder by this treatment was converted into a jelly ; but not 

 completely dissolved. I diluted it with water, and poured the 

 whole on a filter. The white matter that remained on the filter, 

 after being well v/ashed, dried, and heated to redness, weighed 

 18-1 grs. 



To ascertain whether this powder was pure silica, I mixed it 

 with thrice its weight of dry carbonate of soda, and exposed it to 

 a red heat in a platinum crucible. It fused into a liquid which 

 on cooling concreted into an opaque mass having a slight shade 

 of green. Muiiatic acid dissolved it completely. The solution 

 being concentrated assumed the form of a jelly. It was evapo- 

 rated to dryness, mixed with water, and thrown upon the filter. 

 The liquid which passed through the filter was colourless ; but 

 when mixed with carbonate of ammonia, it let fall a slight red- 

 dish coloured precipitate in fiocks, weighing 0*6 gr. Of this 

 one half, or 0-3 gr. was dissolved in caustic potash ley. It was 

 therefore alumina. The remaining 0-3 gr. was oxide of iron. 

 Hence the silica amounted to 17-5 grs. 



The acid solution was precipitated by carbonate of ammonia.. 

 After the white precipitate had subsided, the clear liquid was 

 drawn off, and the white powder was repeatedly edulcorated 

 with distilled water till the liquid drawn off was quite tasteless. 

 All these aqueous liquids were mixed together, and evaporated 

 to dryness. The dry mass was placed upon a sand-bath in a 

 Wedgewood evaporating dish, and exposed to a heat sufficient 

 to volatilize the sal ammoniac of which it chiefly consisted. 

 There remained behind a very small quantity of a white matter. 



