PEOCEEDINGS OF GEOLOGICAL SOCIETIES. 
60 
^ it produces a copious white smoke, due to the formation and deposition 
0 i of silica. If fluor spar and sand be mixed with sulphuric acid, fluoride 
of silicon is formed, which, in passing through water, becomes immediately 
J decomposed, depositing gelatinized silica. If we plunge the delivery- 
• tube, conveying the gas, into water, the quantity of silica immediately 
^ produced would be so great as to stop up the tube, and to burst the 
' flask. In order to avoid that effect, it is usual to place at the bottom 
'> of the vessel a quantity of mercury, over which is the water, and to 
6 let the gas escape in the mercury, and below the water. This gela- 
^ tinous silica, when dry, forms an exceedingly light powder. In fact, 
^ it is in a state of the extremest possible division. As far as I know, tliere 
c 1 is no other way of making amorphous silica in such a fine state of division 
as this. 
1 The solubility of silica is a subject of ver}^ high importance to geologists. 
• I In an aqueous solution of potash, for two parts by weight of solid potash 
c i iri solution there is dissolved one part by weight of this extremely fine 
■ I silica in fluoride of silicon. Of silicon in the state of quartz there is dis- 
1 solved '009 for two parts by weight, and of silica in the state of flint there 
, is dissolved "038. This dilierence is due simply, or at all events in a great 
s measure, to a diflerence of aggregation. When rock-crystal has been 
1 actually melted, and then pulverized, it is as soluble in this menstruum 
I as the silica from the fluoride of silicon. Sdica dissolves, to a certain 
I extent, in water containing alkaline carbonates. The light variety is far 
• I more soluble than the heavy variety. An aqueous solution of carbonate 
[1 : of potash, or soda, for example, dissolves fifteen times more amorphous 
i 1 than crystalline silica. With regard to the solubility of silica in pure 
r water, Bischoff" states, that one part of silica dissolves in 709,230 parts of 
e water. 
5 Some results obtained by Professor Graham are as interesting as they 
e are novel and important, — the phenomena of dialysis, which will possibly 
it hereafter be found to explain mariy obscure geological phenomena. 
Some paper, termed "parchment paper," is tied round a hoop of gutta- 
r percha, forming a circular vessel, the bottom of which consists of parch- 
c ment paper. Into a glass vessel place pure water, and into the hoop 
place a solution of silicate of soda to which acid has been added in such a 
r way as not to precipitate the silica, then place it on the pure water, and 
e leave it. What will take place ? In the course of time a certain propor- 
; tion of the silica will pass through that membrane into the other constitu- 
II ents in the solution, but eventually there will remain in the floating hoop, 
a covered with tiie parchment paper, a pure solution of silica. All the 
hydrochloric acid will be gone by virtue of the operation of that ])aper. 
r The chloride of sodium or potassium, as the case may be, will be gone, — 
• with a certain proportion of silica, it is true, — and there will remain at 
i length a pure, limpid, colourless solution of silica. 
r Amongst the illustrations supplied by Professor Graham for this lecture, 
I, was a 5 per cent, solution of silica. There was no base to retain that 
i silica in solution. It was a pure limpid solution of silica in pure water. 
J In the course of time, if the solution has a certain strength, it will gela- 
i tinize, or, as Professor Graham calls it, pectize — form jelly. The weaker 
1- and purer the solution, the less tendency it has to gelatinize. Professor 
ii Graham expresses an opinion, that with 1 per cent, of silica the solution 
r miglit be preserved for an indefinite length of time without change. 
V There are some very curious properties about this solution to which 
s the lecturer is very anxious to call the attention of geologists. This solu- 
i tion may contain as much as 14 per cent, of silica, and yet be perfectly 
