On the Sullimalion of Silica. 441 



before anv alteration is perceived upon objects which stand only 

 a few inches above it. 



Lynn, Dec, 10, 1814. Ez. Walker. 



[To be continued.] 



LXIX. 0?i the Sul'Iimation of Silica. By J. MacCulloch, 

 M.D. F.L.S. Chemist in the Orclnavce, and I.tclurer on 

 Chemist 11/ ril the Royal Militarij Academy at IFoolwich, 

 r. Pr. Geol. Soc* 



Oo.\iE years aa;o, I)eing in pur.suit of another object, a mi^cture 

 of the oxides of tin and lead was exposed to the heat of an air 

 furnace in an English crucible, to the top of which was luted an- 

 other of the same sort. This a])paratus was; allowed to remain 

 in the fire for some hours. No account of the heat was taken, 

 but I have on former occasions produced in the same furnace a 

 heat sufficient to contract one of ^h•. Wedgwood's original clay 

 pieces to the 130th and 140th degree of his scale. On remov- 

 ing the crucibles, the insides of both from the bottom of the 

 lowermost to within a third part of the top of the uppermost, 

 were found covered with white brilliant filamentous crystals 

 crossing each other in all directions. I concluded that they 

 consisted of the oxide of tin, or perhaps that of lead, and subjected 

 them to the obvious experiments necessary for ascertaining this 

 circumstance. Failing to confirm this supposition, I then con- 

 jectured that thev might consist of silex. The quantity I jiro- 

 cured scarcely amounted to half a grain, and 1 tlierefore divided 

 it into two parts, that I might have the satisfaction of confirm- 

 ing or refuting my own trials by comparison with those of some 

 chemical friend. Mr. Aikin vvas so good as to undertake the 

 examination of the reserved portion, and from his well known 

 accuracy, the Society will naturally place confidence in our mu- 

 tual results. On igniting them in successive portions of borax, 

 and of pure potash, thev were dissolved. The solution was then 

 neutralized, and a few light flakes fell down, which were redis- 

 solved in muriatic acid. This solution being evaporated to a 

 transparent jelly, was ignited by the blow-pipe, and became in- 

 soluble in acids. I was very desirous of obtaining a second spe- 

 cimen, and repeated the same process many times for that end, 

 but iii vain. I cannot pretend to account for this accidental ap- 

 pearance, and only regret that I was unable to ensure it at will. 

 There can be no doubt that they were crystals of silica, however 



* from the Geological Transactions, vol. ii. 



difficuU 



