178 M. C. Deville on the origin of Granite. 



of carbon (which neither I nor M. Pasteur had been able to do), 

 provided that the liquid, heated at least to 80 degrees, undergoes 

 a sudden cooling. 



Without further insisting on these ideas, which more and 

 more confirm my researches made during the last fifteen years, 

 may they not be applied to the consideration of the subject which 

 H. Rose has treated in his memoir? As he justly remarks, 

 the capital fact is that which results from a research of M. Schaff- 

 gotsch* in 1846; that is to say, that quartz in its natural 

 mode of occurrence is in two very different molecular conditions. 

 The one — the crystalline state — has a normal density of 2*65 ; 

 the density of the other does not differ much from 2*2. This 

 notion has been very happily developed by M. Hose, who has 

 shown that these two natural varieties of silica have not the same 

 chemical affinities. The one variety, comprising vitreous quartz, 

 compact quartz, and silex, resists hydrate and carbonate of pot- 

 ash very energetically; while the other variety (opal, whether 

 calcined or not, infusorial silica, &c.) is strongly acted on by these 

 agents. I had also observed in regard to the vitreous quartz ob- 

 tained by temper, that not only had it a density of 2*2, like the 

 natural amorphous varieties, but also that, like them, and per- 

 haps even better than them, it dissolved most readily and almost 

 completely in alkaline lyes. 



This latter fact is closely connected with the preceding expla- 

 nation ; doubtless the very small residue left unattacked by the 

 solution of amorphous quartz in alkaline lyes is comparable to 

 that which the solution of the external coating of pieces of soft 

 sulphur, or even of prisms obtained by fusion, leaves in bisul- 

 phide of carbon. 



As to natural opal, it is evidently formed, as Beudant and 

 Klaproth have long ago shown, from the gelatinous silica which 

 is found still soft in the altered Hungarian trachytes, and the 

 gradual hardening of which may be observed in the suites of 

 specimens in collections. Ebelmen's ingenious researches have 

 shown facts entirely analogous in the silica rapidly precipitated 

 from artificial solutions. " On the other hand," says M. Rose, 

 " the silicic acid of a higher density which constitutes flint, chal- 

 cedony, and crystallized quartz, might be formed from a perfect 

 solution of silicic acid. By the slow concentration of this solu- 

 tion, crystallized quartz might be produced." 



Thu8,in almost all cases,silicic acidof 2*2 spec. grav. maybe con- 

 sidered as resulting from a sudden cooling or rapid precipitation f, 



* PoggendorfFs Annalen, vol. lxvii. p. 147. 



t Silica of organic origin, as that of the infusoria, is doubtless formed 

 under special conditions, which cannot be absolutely assimilated to purely 

 inorganic actions, and in every respect deserves a separate study. 



