258 Dr. Fresenius on an improved Method for the 



completely lost the smell of sulphuretted hydrogen, is filtered 

 upon the smallest possible filter, carefully washed, and while 

 still moist upon the filter drenched with solution of ammonia, 

 and the filter also washed with dilute ammonia until perfectly 

 free from the precipitate. The ammoniacal fluid is then poured 

 into a small porcelain basin, the weight of which has previ- 

 ously been exactly determined, and evaporated in the water- 

 bath ; the residue is dried at 100° C. and weighed. Should 

 the subsequent reduction prove that the residue consisted of 

 arsenic alone, then 0*803 is the amount of arsenious acid to 

 be reckoned for every 1000 of the sulphuret of arsenic ob- 

 tained. Should a residue insoluble in ammonia have remained 

 upon the filter, this must be tested for lead, bismuth, copper, 

 &c. &c. Lead and mercury may moreover be present in the 

 charred residue of C ; this latter also must therefore be fur- 

 ther tested for the presence of these substances. 



E. Reduction of the Sulphuret of Arsenic. 



The separation of the metallic arsenic from the sulphuret 



of arsenic, as the key-stone of the whole investigation, requires 



the greatest care and attention. The apparatus sketched in 



fig. 1 is used for the reduction of the sulphuret of arsenic. 



Fig. 1. 

 a\ i 



A is a capacious flask for the evolution of carbonic acid. It 

 is filled to half its size with rather large pieces of solid lime- 

 stone or marble (not chalk, since this last substance will not 

 yield a continuous stream). To one aperture of the doubly- 

 perforated cork a funnel tube a is adapted, which nearly 

 reaches to the bottom of the vessel ; to the other aperture a 

 tube b, by means of which the gas evolved is conducted into 

 a flask of smaller size, B; in this flask it is washed and dried 

 by the hydrated sulphuric acid contained therein. The tube 



