372 Contributions to Experimental Chemistry. 



transparent jelly. With ^Vth to yj^th of silica, the liquid be- 

 came gelatinous after twelve hours. With 3-iJ^th of silica there 

 was no precipitation of silica even after two days. 



The smallest excess of acid prevents the precipitation of the 

 silica, and solutions containing ^\yth of their weight of silica, 

 did not become precipitated after twenty-four hours. Even 

 boiling could not effect the precipitation of the silica, nor could 

 anything be separated from the liquids by filtration through 

 very dense paper. A solution of silicate of soda, containing 

 ij^th of silica, does not become precipitated by being super- 

 saturated with carbonic acid gas ; but if the carbonic acid is 

 subsequently expelled, either by boiling, or by exposure to the 

 atmosphere, the liquid congeals. When TffW^ of silica is 

 present, neither boiling, nor exposure to the air, nor agitation, 

 cause the precipitation of the silica. Sulphate and muriate of 

 ammonia convert a solution containing -jj^th of silica into a 

 jelly in a few minutes. When the liquid contained ^th of 

 gilica, no immediate change was perceptible ; but after twenty- 

 four hours there was a flaky precipitate of silica, which was 

 still observable when the silica amounted to [^th of the liquid. 



The muriates of lime, magnesia, baryta, and strontia produce 

 an immediate precipitate in a solution of silicate of soda, 

 containing j^fii of silica. With ^fii of silica no precipitate 

 ensues. 



If a solution of silicate of soda be gradually poured into a 

 solution of perchloride of iron, the precipitate so produced is 

 instantly redissolved, and some of the ^lica is only then sepa- 

 rated when the solutions are too concentrated. If the solution 

 of silicate of soda contain about ^th of silica, no precipitation is 

 perceptible ; the colour of the solution of the perchloride be- 

 comes more intense, and after the mixture has been boiled, so 

 much so, that one grain of the perchloride is able to colour 

 20,000 grains of water, distinctly yellow. 



If the mixture contain but |ths proportional of silicate of soda 

 (= six proportionals for every proportional, upon the whole 

 less than twelve proportionals of silica for every proportional 

 of perchloride of iron), it reacts on test-paper as an acid : ferro- 

 cyanate and sulpho-cyanate of potash, and tincture of galls, 

 indicate the presence of iron. Sulphuretted hydrogen very 



