(J34 ARSENIC. 



3. Wash the precipitate by affusion of water acidulated 

 with hydrochloric acid, and subsidence. 



4. Dry the precipitate at a temperature not exceeding 

 300. 



III. Reduction of the sulphuret of arsenic : 



1. Mix the dried precipitate intimately with twice its 

 bulk of dry black flux (carbonate of potash and char- 

 coal), and heat to redness in a glass tube, of the form 

 and size of a or b, exhibited below. 



2. Heat slowly a particle of the metallic crust in a glass 

 tube Cy and observe the formation of a white crystal- 

 line sublimate of arsenious acid. 



FIG. 82. 





Hydrogen cannot be evolved in contact with a soluble or in- 

 soluble preparation of arsenic, without combining with the 

 metal, which is thus removed from the liquor, in the form of 

 arsenietted hydrogen gas. Mr. Marsh has founded, upon that 

 fact, a simple and elegant mode of obtaining metallic arsenic 

 from arsenical liquors. The stopcock being removed from the 

 FIG. 83. bulbed apparatus represented in the figure, a 

 fragment of zinc is placed in the lower bulb, 

 and diluted sulphuric poured upon it. The 

 stopcock being replaced and closed, the lower 

 bulb is soon filled with hydrogen gas, and the 

 acid liquid forced into the upper bulb. It is 

 necessary to test this hydrogen for arsenic, 

 which will be found in it, if the zinc itself con- 

 tains that metal. The gas for this purpose is 

 kindled at the stopcock and allowed to burn 

 with a small flame. If a stoneware plate be 

 depressed upon the flame, a black spot of a 

 steel-grey colour and bright metallic lustre, 



