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



described in his " Two Processes,"* cyanide of lead was found by Dr BROWN to 

 resolve itself entirely into nitrogen, which escaped in the gaseous form, and a 

 bluish-grey powder, which, when digested in dilute nitric acid, yielded its lead to 

 that solvent, and left silicon as an insoluble brown powder. The general precau- 

 tions indicated as essential to the success of the transmutation were easily attended 

 to, and the only point which was much insisted on, was the necessity for the cyanide 

 of lead being absolutely pure. 



The realization of this desideratum proved much more difficult than we had 

 at all expected ; so difficult, indeed, that, after six weeks spent in unsuccessful at- 

 tempts to prepare a pure cyanide of the metal in question, we abandoned, for the 

 time, the process in despair. 



The method given by Dr BROWN in his Two Processes, was to precipitate the 

 cyanide of potassium by the neutral acetate of lead. By this process, however, 

 and by another similar in principle, and likewise employed by that gentleman, in 

 which ammonia, supersaturated with hydrocyanic acid, was substituted for cyanide 

 of potassium, we did not succeed in preparing a pure cyanide of lead. The white 

 precipitate which fell when these reagents were made use of, was found on analysis 

 to give a proportion both of lead and of cyanogen, quite at variance with the pos- 

 sibility of its being the pure protocyanide ; the average proportion of cyanide of 

 lead present being, as nearly as possible, only a third of its whole weight. When 

 distilled with oil of vitriol in a water bath, it gave off acetic as well as hydrocyanic 

 acid ; and when heated alone in a tube, cyanide of ammonium was evolved in 

 large quantity. 



We did not determine the exact composition of this compound, as it was suffi- 

 cient for our purpose to know that it was not the salt we were in search of. But 

 it appeared to consist of cyanide of lead mixed with a large proportion of a hy- 

 drated basic acetate of the same metal. 



Besides the neutral acetate of lead, we employed that salt acidulated with 

 acetic acid, the tribasic acetate, the nitrate, the basic nitrate, and the nitrite, as 

 precipitants of the alkaline cyanide ; and we varied the proportions of hydrocyanic 

 acid and ammonia, and all the minor details of the several processes, without at- 

 taining the end we had in view. In every case the tendency of lead to form basic 

 salts, gave rise to the mixture of the cyanide produced, with subacetate and subni- 

 trate of the oxide of the metal. 



We also digested cyanide of potassium in solution on ferrocyanide of lead, in 

 the expectation of removing ferrocyanogen, and leaving cyanide of lead, but the 

 process did not succeed. Nor were we more successful with another, where we 

 digested hydrated oxide of lead in dilute hydrocyanic acid. 



Finally, we had recourse to the iodide and chloride of lead, which we dissolved 



* Two Processes for Silicon, by Dr SAMUEL BBOWN. Edinburgh : Adam and Charles Black. 1843. 



