Reports and Proceedings. 221 
tion. By bringing, however, to our aid a form of solution of silica 
of comparatively recent discovery, we might arrive at a possible 
theory on this subject. The solution of silica he referred to was 
one of the many products of Dr. Graham’s process of dialysis, by 
which all substances soluble in water are capable of being divided 
into two classes—Crystalloids and Colloids. Silica belongs to the 
latter class ; and the difficulty of obtaining a strong solution of it, 
prior to Dr. Graham’s discovery, was the impossibility of getting 
rid of the crystalloid substances always accompanying its solution. 
By the process of dialysis, however, a solution of pure silica can 
readily be obtained, and which, after being condensed by evapora- 
tion, gelatinizes, and, when dried under the receiver cf an air-pump, 
is, in fact, artificial hyalite, or opal, having the constitution and all 
the properties of that mineral. ‘This is a pure amorphous variety of 
silica, with the specific gravity of 2°3; and the argument in favour 
of flint, chalcedony, and the like, having a crystalline structure, is 
the fact of their specific gravity being the same as that of the crystals 
of quartz, 2°6; but then we must take into consideration the fact 
that our amorphous variety is prepared under the ordinary pressure 
of the atmosphere, whereas flints, the quartz of granites, and other 
varieties of silica, having the high specific gravity, are, in all 
probability, formed under great pressure, which may sufficiently 
account for their higher density. Silicates of soda or potash are 
infinitely more soluble under great pressure than in the atmosphere ; 
- hence beneath the strata of the earth’s crust a more concentrated 
solution of silica would doubtless be produced. We have silicates 
of soda and potash, and we have also hydrochloric and carbonic 
acids often freely evolved, and proving their capability of liberat- 
ing the silica; and that this may be the mode of production of 
many of the pure silica minerals, and especially of quartz, might 
be safely assumed; at all events, from a chemical point of view, 
it seems the most likely theory. But then, how are we to get 
rid of the chloride of sodium produced along with our quartz, and 
incorporated in its structure? Dr. Percy has suggested that pos- 
sibly beds of sandstone may have a dialytic action, separating col- 
loid from crystalloid substances, by which quartz would be formed 
from a silicate of soda or potash, as already explained. From an 
experiment made by Mr. Sutherland, he thought this dialytic me- 
dium was not required. The gelatinous silica artificially produced 
is itself permanently dialytic, and appears to possess a contractile 
power, by which, combined with the dialytic, it throws out all ex- 
traneous salts. He had laid aside a piece of it in a dry place, when 
the salt it contained at once began to effloresce from its surface, 
while it continued to shrink up into a substance having the hardness 
of common resin, and not unlike the cairngorm in appearance: 
it continued to shrink even after it had become comparatively hard; 
and although reduced to one-fourth of its original mass, it retained 
its shape entire. Metallic oxides brought into contact with this 
gelatinous silica, produced in it the colouring and variegated ap- 
pearance that characterize many of our pebbles; there was also a 
