22 



EEOEEATIVE SCIENCE. 



fixing water-glass — each, having its appro- 

 priate qualities and uses. Potash, water- 

 glass is made by fusing together, for five 

 or six hours, at a high temperature, in a 

 fire-proof pot, a mixture of 



15 parts of pulverized quartz or pure quartz 

 sand (otherwise silica, or silicic acid), 



10 parts of well-purified potash, 

 1 part of powdered charcoal. 



The charcoal is said to decompose any sul- 

 phuric acid left in the potash, and so a perfect 

 vitrification of the mass is obtained. The 

 hard, blistered, grayish black mass thus 

 obtained is pulverized, then boiled with five 

 times its weight of water, in which it slowly, 

 but almost entirely, dissolves, in the course 

 of a few hours. The solution is finally 

 evaporated. With a larger proportion of 

 silica, as pure rock-crystal, quartz, sand, or 

 flint are termed, an insoluble glass, resembling 

 ordinary glass, is the result. 



Water-glass may also be obtained by 

 Messrs. Hansome's method of dissolving 

 broken flints in a solution of cUustic alkali, 

 at a temperature of 300'^ Fahr. Or an 

 aqueous solution of potash may be saturated 

 with freshly precipitated hydrate of silica, 

 and evaporated. Or a fourth method may 

 be employed, as indicated by Mr. Way, who 

 dissolves a peculiar kind of sand, which he 

 has described, at the ordinary boiling heat, 

 in a solution of caustic alkali (potash or soda). 



In order to obtain the mass prepared by 

 either of these methods in the anhydrotis state, 

 it must be heated till it fuses, when all water 

 is expelled. It is then a hard, transparent, 

 rather infusible glass, which, on exposure to 

 the air, absorbs so much water (without any 

 external change, excepting that it becomes 

 slightly fissured), that it swells up strongly 

 when heated. 



Its composition, when made with potash. 



u- 



Potash . . . . . . 27-57 



Silicic acid, fourcombiniug proportions 72'43 



100-00 



An analysis, by Forchammer, established the 

 theoretical statement of its constitution. 



When fully charged with combined water 

 it is termed hydrated water-glass. Simply 

 drying the solution of water-glass yields this 

 hydrate, which is colourless, transparent, 

 and brittle, but softer than glass. It is 

 slightly alkaline, and, after thorough drying, 

 contains 26 per cent, of potash, 62 of silica, 

 and 12 of water. The salt is permanent in 

 the air ; does not absorb carbonic acid from 

 it; and effloresces only when accidentally 

 mixed with other salts of potash. In the 

 fire it swells up with loss of water, then 

 fuses, and forms anhydrous soluble glass. 

 Dilute acids decompose it, with separation 

 of silica. It dissolves but slowly in cold, 

 but readily in boiling water. After eva- 

 poration, it becomes very tenacious, 

 and may be drawn out in threads, like 

 melted glass. It dries up to a varnish, when 

 spread upon wood, paper, linen, etc., the 

 combustibility of which it diminishes. Sal 

 ammoniac precipitates silica immediately 

 from its solution. Alcohol also precipitates 

 the soluble glass from its aqueous solution, 

 withdrawing, however, some of the potash, 

 until, at last, on washing the precipitate, 

 octo-silieate of potash alone remains. 

 Phosphate of alumina, and carbonate, 

 phosphate, or sulphate of lead, when rubbed 

 lip with a solution of soluble glass, yield a 

 tenacious mass, which becomes as hard as 

 stone in the air. Baryta, strontia, lime, alu- 

 mina, and oxide of lead combine with the 

 whole of the silica and a part of the potash, 

 to form an insoluble compound. Nearly aU 

 the soluble salts of the earths and heavy 

 metallic oxides produce a btdky precipitate. 

 These details of its chemical behaviour will 

 enable the experimental reader to carry on 

 those further investigations, which Fuchs so 

 earnestly desired to see accomplished. The 

 writer of this notice has proved that positive 

 and negative photographs withstand suffi- 

 ciently the action of boiling caustic alkali. 



