36&^ Dr. Thomsofi on the [Nov. 



(4.) All this liquid that had passed throug;h the filter (pro- 

 perly concentrated) was again poured into the original flask, 

 and a solution of carbonate of soda was added to it till the 

 liquid contained a decided excess of alkali. This new alkahne 

 liquid occasioned the appearance of a new precipitate which 

 was in white light flocks, like the carbonate of zinc which had 

 been thrown down cold. After this precipitate had subsided, 

 the flask was heated by a spirit lamp and kept boiling for about 

 20 minutes ; the whole was then poured upon the same double 

 filter upon which the second precipitate (6) had been collected ; 

 and the filter was washed with distilled water till it ceased to 

 produce any alteration on muriate of barytes. 



The precipitate collected on this filter was now dried on the 

 filter and weighed. Its weight was 6*07 grains. After exposure 

 to a red heat it was reduced to 4-54 grains. 



(5.) The liquid thus freed from all the oxide of zinc that could 

 be thrown down by boiling was put into a porcelain dish, and 

 slowly evaporated to dryness on the sand bath. The dry resi- 

 dual salt being redissolved in water, a few flocks of oxide of zinc 

 separated. These collected and dried on the filter weighed 0*44 

 grain, and when heated to redness were reduced to 0*431 grain. 



(6.) The liquid containing the residual sulphate and carbonate 

 of soda was again evaporated to dryness in a platinum vessel, 

 and the dry residue exposed for an hour to a strong red heat. 

 The salt thus treated when dissolved in water deposited a few 

 black flocks. These being collected and exposed to a red heat 

 in a platinum spoon became grey, and weighed 0*3 grain. Being 

 digested in nitromuriatic acid, the bulk diminished, and a por- 

 tion was dissolved. The solution was colourless, and was preci- 

 pitated in white gelatinous flocks by prussiate of potash, showing 

 that it was oxide of zinc. The undissolved portion was not acted 

 on by any acid, but it fused before the blowpipe with carbonate 

 of soda into a white globule, and the solution was accompanied 

 wjth effervescence. Hence I considered it as silica ; conse- 

 quently the precipitate was silicate of zinc, and it contained 0*22 

 grain of oxide of zinc. 



I do not know the cause of the black colour which this powder 

 had at first. It seems to have been owing to the presence of 

 some combustible matter, as it was dissipated by heat. The 

 platinum crucible was covered with a lid all the time that it 

 was in the fire, so that no charcoal could have reached it from 

 the fuel. 



The carbonate of soda used was pure, consisting of crystals 



Sicked with great care from Mr. lennant's evaporating pans, 

 [ence I am disposed to ascribe the origin of the silica to the 

 glass flask in which the mixture had been so long boiled. 



(7.) The solution from which the silicate of zinc had been 

 deposited was neutralized with muriatic acid, and then mixed 



