DETKCTION OE ARSENIC. 129 



as its characteristics are well defined, it is easily applied, and 

 but few precautions are necessary for its use. Should the 

 fluid to be examined by it contain any free acid, it must first 

 be neutralized by potassa, and then slightly acidulated by a 

 few drops of acetic or muriatic acid. When the fluid is thus 

 prepared, a stream of the gas, is to be passed through it, for 

 ten or fifteen minutes, or the water saturated with gas is 

 to be poured into it. The eflect of this test is, if the arsenic 

 be present in sufficient quantity, to produce a bright lemon 

 precipitate, or if the quantity be exceedingly small, to change 

 the colour of the solution to a bright lemon colour. To 

 insure the full separation of the precipitate the solution must 

 be boiled, because the sulphuret of arsenic is soluble in an 

 excess of sulphuretted hydrogen. This test is liable to a few 

 fallacies. The salts of cadmium are precipitated by it of a 

 very similar colour, but these are exceedingly rare, besides 

 which, the sulphuret of cadmium is soluble in muriatic acid, 

 and insoluble in ammonia, in this respect difl'ering from the 

 sulphuret of arsenic. The salts of selenic acid under certain 

 circumstances yield yellow precipitates with sulphuretted hy- 

 drogen, but these salts are exceedingly rare. The per salts 

 of tin, which are of almost equally rare occurrence, in any 

 case requiring a medico-legal investigation, yield yellow pre- 

 cipitates with this test, but they are turned brown by am- 

 monia. It has been objected, that antimonial salts, when 

 treated by sulphuretted hydrogen, give yellow coloured pre- 

 cipitates, but their colour, as is well known to chemists, are 

 of an orange red hue, very distinctly recognized, and distin- 

 guished from sulphuret of arsenic. Tlie presence of any acid 

 in the solution, except acetic and muriatic, will produce a 

 dirty yellowish white coloured precipitate, with sulphuretted 

 hydrogen, caused by the separation of the sulphur, but this is 

 easily distinguished from sulphuret of arsenic. When the 

 arsenic exists in the state of arsenic acid, or one of its salts, 

 after the sulphuretted hydrogen has been added to the solu- 

 tion, it is necessary to boil it, and allow it to stand by for some 

 time, before the precipitate forms. Dr. Christison says, that 

 he cannot consider the process he recommends in complex 

 fluids as competent to develope the presence of arsenic acid ; 

 from a careful examination of bis process, I cannot discover 

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