deposited from aqueous solutions ; his examination of the 
Koh-i-noor diamond and others in the East India Company’s 
museum; and the geological speculations to which these 
discoveries gave rise. Mr. Dancer mentioned the experi- 
ments of his late father and others in producing artificial 
gems by intense heat, and stated that his own attention was 
drawn to this subject some twenty-four years since, by Sir 
David Brewster presenting him with a specimen of topaz con- 
taining fluid, Since that time he had examined a large 
♦ 
number of crystals, of various kinds, from the collections of 
friends ; and had found fluid in quartz from South America, 
Norway, the Alps, Ireland, Snowdon, and the Isle of Man; 
and in fluor spar from Derbyshire; this latter specimen 
contained a considerable quantity of fluid, which burst the 
crystal at 180° temperature.* He suggested the employment 
of the microscope as a valuable assistance in detecting 
spurious from real gems ; very few of the latter are perfect, 
and the flaws and cavities are so distinct in character from 
those which are so abundant generally in artificial gems that' 
very little experience is sufficient for the purpose, This 
mode of testing of course is limited to transparent crystals, 
but might be employed when the usual methods are not 
practicable. He also mentioned Mr. Sorby’s (F.R.S.) dis- 
covery of fluid cavities in the quartz of granite, in the quartz 
of volcanic rocks, and also in the feldspar ejected from the 
crater of Vesuvius, and Mr. Sorby’s method of determining 
the temperature at which various rocks and minerals are 
* After this Paper was written, Sir David Brewster informed the Author 
that the fluid contained in crystals of fluor spar was water, and that the 
cavities burst at a temperature of 150°. 
