38 Proceedings of the Royal Society 
submit them to a cooling process extending over some twelve or 
fourteen hours. 
Whilst pursuing the other experiments, on dissolving the silver 
afterwards in nitric acid, it yielded from its interior a number of 
very beautiful laminae or leaf-shaped crystals of the hexagonal 
system, and varying in colour from light yellow to dark brown or 
even black, which at first sight I mistook for graphite, formed by 
the solution of the carbon in the silver and crystallizing out from it 
in this characteristic form. 
There were also present a number of other crystals in the form 
of hexagonal prisms or crystalline aggregates, but these were in 
many, if not in all cases, perfectly colourless and transparent. It 
may be menticned that some of them appeared to possess the 
curved form sometimes met with in quartz, 
and given in fig A. 
Neither hydrochloric acid nor nitric acid 
had any action upon them, but on treating 
them with boiling hydrofluoric acid for some 
time, they quietly dissolved, and the same 
thing occurred when they were boiled with 
a strong solution of caustic potash. 
This perplexed me at first with regard 
to the leaf-like forms, for it was perfectly 
clear that they were not graphite, and I could not find that silica 
was known to crystallize in that manner. It was evident the silver 
must therefore have been in contact with the porcelain crucible, and 
these crystals in this way derived from it. 
The thing to be done then was to have some microscopic 
sections made of the different crucibles, — first of one which had 
not been heated, and then of those which had been used in the 
experiments in order to see what effect the heat had had upon 
them. In this way it was possible to compare the two side by side, 
and what I found was this, that the alumina portion of the crucible 
had undergone little or no change, but that the glaze of the crucible, 
consisting of silica, which, in the original condition, was in a per- 
fectly homogeneous and vitreous state after the heating, had become 
one mass of little crystals in the form of hexagonal prisms. Here 
then was a clue to what these unknown crystals were, namely, silica. 
