276 Dr. Mac Culloch oji an Accidental Sublimation of Silica. 



them in successive portions of borax and of pure potash, they were 

 dissolved. The solution was then neutralized, and a fev/ light 

 flakes fell down, which were redissolved in muriatic acid. 

 This solution being evaporated to a transparent jelly, was ignited 

 by the blowpipe, and became insoluble in acids. I was very 

 desirous of obtaining a second specimen, and repeated the same 

 process many times for that end, but in vain. I can not pretend to 

 account for this accidental appearance, and only regret that I was 

 unable to ensure it at will. There can be no doubt that they were 

 crystals of silica, however difficult we may find it to form them at 

 pleasure, and the rarity of the occurrence only serves to prove that 

 there are properties and relations of this substance with which we 

 are as yet unacquainted. An agreeable confirmation of this fact ap- 

 peared some time after In an observation of Vauquelin, copied in 

 Tilloch's journal for 1809, with which the members of this Society 

 arc doubtless well acquainted. In a geological view it may per- 

 haps be worthy of record as not only establishing the volatility of 

 silica, but serving to prove that this substance may be crystallized 

 from the state of vapour, as sulphur, some neutral salts, and some 

 metals are known to be. How far this property of vaporization and 

 crystallization from that state may be possessed by the other earths, 

 or by earthy compounds, as It undoubtedly is by all the metals, must 

 be determined by future observations. Possibly we may thus gain a 

 step on which to rest, in the investigation of the difficult subject of 

 mineral veins, and the arrangement of the crystallized substances 

 which occupy their cavities. The possibility also of explaining by 

 this process the crystallization of the delicate filamentous zeolites 

 which occupy the cavities of amygdaloids, will readily occur to every 

 mineralogist. 



