Iodide of Pot as sum, as a Test for A'rsenic. 



61 



The process which was found best calculated to furnish the compo- 

 sition, consists in decomposing the powder by a solution of the am- 

 monio-nitrate of silver, cautiously added, until it ceases to occasion 

 any precipitation. There are thus formed an iodide and arsenite of 

 silver nearly insoluble in water, while the solution contains, (in union 

 with nitric acid,) all the alkali originally present in the precipitate. 

 The soluble and insoluble portions were separated by decantation. 



From the former, all excess of nitrate of silver was precipitated 

 by a few drops of muriatic acid, and the solution then evaporated to 

 dryness — lastly, the nitrate was converted into a sulphate, and all ex- 

 cess of matter driven off by a red heat — from the sulphate, the alkaU 

 was determined both directly and by means of nitrate of barytai 



From the latter, the arsenite of silver was removed by dilute nitric 

 acid, and, after separation, converted into a chloride, by the addition 

 of common salt. By such experiments, the iodide varied from 23 

 to 28 per cent., but the arsenious acid proved very indefinite, and 

 always very far exceeded the amount necessary for combination with 

 the alkali obtained, even to the exclusion of the iodine. The follow- 

 ing experiments were made in order to determine how far the ar- 

 senious acid contributes to this irregularity. An arsenite of potassa 

 was prepared, according to the usual directions, by boiling carbonate 

 of potassa with excess of arsenious acid. The solution was allowed 

 to cool, and then filtered. Still, upon concentration, arsenious acid 

 was regularly deposited, thus indicating that the arsenite was not neu- 

 tral, owing to the power which Its solution had of dissolving the acid. 

 Cold water was next added to dissolve the salt, the arsenious acid being 

 almost Insoluble at this temperature, and the solution was again con- 

 centrated. Upon the addition of a drop of acetic acid, a white cloud, 

 owing to the separation of the arsenic, appeared, but upon agitating 

 the mixture, again immediately disappeared. This precipitation and 

 re-solution was repeatedly performed by successive drops of acetic 

 acid, and ceased only when the latter had completely decomposed 

 the arsenite ; then the arsenious acid separated copiously and per- 

 manently, adhering, with great firmness, to the glass vessel, and 

 marking the course of the rod with white streaks. In these experi- 

 ments we perceive clearly the facility with which this acid, notwith- 

 standing its insolubility, may be retained in solution, and to this cir- 

 cumstance may be attributed the variable quantity furnished by the 

 precipitate with iodide of potassium. It is difiicult moreover to evap- 

 orate the solution of arsenite of potassa to perfect dryness without 



