PROCEEDINGS OE GEOLOGICAL SOCIETIES. 



69 



it produces a copious white smoke, due to the formation and deposition 

 of silica. If fluor spar and sand be mixed with sulphuric acid, fluoride 

 of silicon is formed, which, in passing through water, becomes immediately 

 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 

 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, there 

 is no other way of making amorphous silica in such a fine state of division 

 as this. 



The solubility of silica is a subject of very high importance to geologists. 

 In an aqueous solution of potash, for two parts by weight of solid potash 

 in solution there is dissolved one part by weight of this extremely fine 

 silica in fluoride of silicon. Of silicon in the state of quartz there is dis- 

 solved - 009 for two parts by weight, and of silica in the state of flint there 

 is dissolved "038. This difference is due simply, or at all events in a great 

 measure, to a difference of aggregation. When rock-crystal has been 

 actually melted, and then pulverized, it is as soluble in this menstruum 

 as the silica from the fluoride of silicon. Silica dissolves, to a certain 

 extent, in water containing alkaline carbonates. The light variety is far 

 more soluble than the heavy variety. An aqueous solution of carbonate 

 of potash, or soda, for example, dissolves fifteen times more amorphous 

 than crystalline silica. With regard to the solubility of silica in pure 

 water, Bischoff states, that one part of silica dissolves in 769,230 parts of 

 water. 



Some results obtained by Professor Graham are as interesting as they 

 are novel and important, — the phenomena of dialysis, which will possibly 

 hereafter be found to explain many obscure geological phenomena. 



Some paper, termed " parchment paper," is tied round a hoop of gutta- 

 percha, forming a circular vessel, the bottom of which consists of parch- 

 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 

 way as not to precipitate the silica, then place it on the pure water, and 

 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- 

 ents in the solution, but eventually there will remain in the floating hoop, 

 covered with the parchment paper, a pure solution of silica. All the 

 hydrochloric acid will be gone by virtue of the operation of that paper. 

 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 

 length a pure, limpid, colourless solution of silica. 



Amongst the illustrations supplied by Professor Graham for this lecture, 

 was a 5 per cent, solution of silica. There was no base to retain that 

 silica in solution. It was a pure limpid solution of silica in pure water. 

 In the course of time, if the solution has a certain strength, it will gela- 

 tinize, or, as Professor Graham calls it, pectize — form jelly. The weaker 

 and purer the solution, the less tendency it has to gelatinize. Professor 

 Graham expresses an opinion, that with 1 per cent, of silica the solution 

 might be preserved for an indefinite length of time without change. 



There are some very curious properties about this solution to which 

 the lecturer is very anxious to call the attention of geologists. This solu- 

 tion may contain as much as 14 per cent, of silica, and yet be perfectly 



