39 
of Edinburgh, Session 1880-81. 
The prismatic crystals were evidently derived from this source, 
and simply occurred diffused through the silver by having been 
taken in through the action of convection currents, which still 
operated — after they had been produced through the cooling of the 
silica to the temperature necessary for their formation — whilst the 
silver yet remained in a perfectly liquid condition. 
The leafdike crystals, it was easily to be conceived, were from 
the same source ; but the question naturally arises, as to what led 
the portion of the silica of which they were composed to crystallize 
in this peculiar graphite-like form instead of in its usual prismatic 
one, as it had done in the glaze of the crucible, where it had not 
been in contact with the metal ? 
To account for this, it is conceivable that here we had silica in 
quite a different state from what it is when heated by itself — 
namely, in a state of solution , in the silver (not suspension), and 
that in this condition of solution it is capable of undergoing what 
we may designate as a molecular disaggregation, owing perhaps to 
some influence of the silver, and thus to be capable of arranging 
itself in these new forms. 
At first I thought that possibly these crystals might be composed 
of silicide of silver, for it is stated (“ Watts’s Dictionary,” vol. v. 
p. 241) that “ silica is decomposed by carbon in the presence of 
silver at a white heat, carbonic acid and a silicide of silver being 
formed.” 
Be this as it may, these experiments do not appear to confirm this 
statement ; for although they were conducted at a temperature a 
little above the melting point of steel, yet, on testing the solution 
of these crystals for silver, it was not possible to detect the slightest 
trace of that metal. 
Their colour I believe to be due either to amorphous-carbon in 
a state of extremely minute division being disseminated through 
them, or perhaps to the presence of a very slight trace of iron 
which might be derived from the other part of the crucible. Accom- 
panying this paper are some sketches showing the beautiful form 
and colour of several of these crystals, and in which they are 
enlarged about 320 diameters. 
A study of the crystallization of silica from iron, and the effects 
which it produces in so doing, seems likely to yield some results of 
