552 ACCOUNT OF A REPETITION OF SEVERAL OF DR SAMUEL BROWN'S 



30 grs. of the ferrocyanide ; and the collected product of several others gave 

 1240 grs. of silica from 9334 grs. of the prussiate.* 



We repeated this experiment many times, both in platina and iron crucibles, 

 but never could obtain more than traces of silica, although we employed large 

 quantities of material. The proportion of silica procured in our early trials was so 

 insignificant, that we did not attempt to ascertain its quantity. But in a recent 

 experiment, where 480 grs. of ferrocyanide of potassium were fused with 3 ounces 

 of bicarbonate of soda (which in this case was substituted for potass on account of 

 its purity), the quantity of silica obtained was only 0*3 gr., or less than a tenth of 

 a grain for each ounce of material, soda included. 



We tried a single experiment with the variety of Prussian blue, formed by 

 exposing to the air and washing the salt left in the retorts, when ferrocyanide of 

 potassium is decomposed by sulphuric acid in the process for the preparation of 

 prussic acid. 619 grs. of this body were fused with three times their weight of 

 carbonate of soda, and treated otherwise like the ferrocyanide of potassium. At 

 the stage of the process where silica should have appeared, we obtained 12 grs. of 

 a soft yellowish-white insoluble powder, which, in the belief that it would prove to 

 be silica, we ignited for two hours in a platina crucible. It was not silica, how- 

 ever, but probably some organic compound ; for the crucible, on being opened, was 

 found quite empty. 



From the ferrocyanide of lead, on which we made several experiments, we ob- 

 tained traces of silica. 



From the ferrocyanide of copper fused with alkaline carbonates, and treated 

 otherwise like the similar salt of potassium, we obtained a larger amount of silica 

 than from any other salt of the same class. From 266 grains of a parcel of this 

 salt, prepared by one of us with great care, and fused with three times its weight 

 of white flux, we obtained 4'2 grs. of silica ; and other portions of the same spe- 

 cimen yielded us considerable, though variable, quantities. 



These experiments were made early in the winter, at a time when we were 

 only anxious to ascertain if silica could be procured from such materials. In many 

 of them, accordingly, unweighed quantities of the ferrocyanide and the alkaline 

 carbonate were employed. As, however, we preserved nearly all the silica we 

 procured in this way, and are able to form an estimate of the quantity of the ferro- 

 cyanide made use of, from the space it occupied in a bottle of known capacity, we 

 can make a tolerably near approximation to the weight we must have employed. 

 On the whole, we are within the truth when we say, that from about three ounces 

 of ferrocyanide of copper, fused with twelve of alkaline carbonate, we obtained 

 ten grains of silica. The carbonate was ascertained by very careful analyses to be 

 quite free from silica, except in one case, where, owing to an accidental oversight, 



* Transactions of the Royal Society of Edinburgh, vol. xv. p. 245. 



