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groups of granites, in addition to the most important differ- 
ence, to which I have drawn attention, with respect to their 
alkaline constituents; but I content myself at present with 
establishing this fundamental distinction between the two 
groups, and thus furnishing an additional proof of the service 
which may be rendered to geological science by the more exact 
and experimental sciences.” 
Mr. Robert T. Forster, by permission of the Council, read 
a paper on the molecular formation of crystals: he first gave 
an account of preceding theories on this subject, and having 
shown in what respect these different theories failed, he pro- 
ceeded to explain by what means we can account for the occur- 
rence of secondary forms in general, and also advanced an hy- 
pothesis by which the formation of most hemihedral forms can 
be satisfactorily demonstrated, and the circumstances under 
which the various changes take place can be clearly pointed 
out. 
‘¢ The first writer whom we find deserving of notice is Huy- 
gens, who considered the crystals of Iceland spar to be built 
up of spheroids. He did not, however, give any explanation 
why these spheroids are so aggregated. 
*¢ Hooke, in his ‘ Micrographia,’ advanced a similar hypo- 
thesis, except that he considered the atoms to be spherical, a 
supposition which is utterly inapplicable to the third or rhom- 
bohedral system to which he applied it. 
‘‘The next writer who commanded attention was M. 
Prechtl; he considered a fluid to be made up of soft molecules, 
which, while the body was undergoing its change of state, 
suffered a change of form, arising from their mutual pressure, 
and that under different degrees of compressibility different 
forms were produced. Not only was this hypothesis quite 
insufficient, but it was also erroneous in a mathematical point 
of view, as Dr. Wollaston has fully shown. 
‘‘ Immediately after him Dr. Wollaston published, in ‘ The 
a 
