36 = Prof. H. Rose on the different States of Silicic Acid, 
Thus, en résumé, we are obliged to admit the existence of two 
distinct modifications of silica—one amorphous, the density of 
which varies from 2°2 to 2:3 ; the other crystallized, the density 
of which is 2:6. This last is formed in the humid way alone, 
or at least in presence of water; the first is produced either in 
the humid way or by fusion. Crystallized silica is the only 
kind found in granite. 
It is impossible not to be struck with the importance of these 
facts for the discussion of the theories proposed to account for 
the origin of granites. Accordingly the author treats this im- 
portant question in detail. 
It is known that the theory of Werner on the Neptunian 
origin of granite was afterwards completely abandoned by geo- 
logists, and replaced by the Plutonic theory. The latter, however, 
has always met with adversaries, especially among chemists, and 
seems to have been shaken more and more during the last few 
years. MM. Fuchs, G. Bischoff, and more recently M. Delesse, 
particularly deserve to be mentioned as having most contributed 
to raise doubts as to the tenability of this theory. 
Fuchs especially bases his objections upon the simultaneous 
presence in granite of minerals the points of fusion of which are 
extremely different, and upon their reciprocal penetration, which 
does not permit one to doubt their simultaneous formation. He 
attributes also considerable weight to the absence, in granite and 
analogous rocks, of vitreous matter which ought to be found in 
the productions of an igneous fusion. 
Bischoff was in like manner led to reject the hypothesis of the 
igneous origin of granite by the fact that in this rock felspar, an 
element rather fusible, is generally encrusted in mineral quartz, 
almost infusible ; the felspar, consequently, solidified first, which 
is inexplicable on the hypothesis of a crystallization produced by 
the cooling of a melted mass. But he forms his opimion also on 
the considerations drawn from the study of all the elements of 
granite. 
One of the elements of granite, felspar, can be artificially ob- 
tained either in the humid or dry way. Daubrée has succeeded in 
producing a crystallized felspar similar to that of the trachytes by 
the action of water on obsidian or clay in the presence of an alkali, 
under the influence of a high temperature and great pressure. 
With respect to its production by means of ignition, it was 
accidentally observed in the remains of a copper furnace at 
Sangershausen, but we have not yet succeeded in obtaining it at 
will; by the fusion of felspar or its elements and by a slow cool- 
ing, only vitreous matters have been obtained. This formation 
of felspar is therefore possible, but it seems to require a com- 
bination of circumstances difficult to realize. 
