ARSENIC AND LEAD. 77 



The solution filtered from the magnesia-precipitate 

 is acidified with hydrochloric acid, and the arsenious 

 acid precipitated as tersulphide of arsenic. 



If the alkaline solution is saturated with hydro- 

 chloric acid, and heated with sulphurous acid, the 

 arsenic acid is reduced to arsenious acid and may be 

 precipitated by hydrosulphuric acid. Or if the solu- 

 tion is saturated with chlorine gas, all the arsenious 

 acid will be converted into arsenic acid, and may then 

 be precipitated by the ammoniated salt of magnesia.* 



If the original pigment be digested with a mixture 

 of concentrated hydrochloric acid and alcohol, a solu- 

 tion of chloride of copper is obtained, and the arsenious 

 acid remains behind as a white powder. 



By long digestion with an excess of sulphide of am- 

 monium, the copper is separated as sulphide, while all 

 the arsenic is dissolved and may be precipitated as 

 tersulphide of arsenic by adding hydrochloric acid to 

 the filtrate. 



When Schweinfurt green is distilled with dilute sul- 

 phuric acid, the acetic acid passes over, and may be 

 converted into acetate of baryta by adding that base. 

 The accurate quantitative estimation of the acetic acid 

 can only be effected by ultimate organic analysis. 



52. ARSENIC AND LEAD. 



The alloy, in a fine state of division, is oxidized with 

 nitric acid, the excess of acid evaporated, the solution 

 neutralized with ammonia, and the precipitated white 

 mass digested for some time with an excess of sulphide 

 of ammonium in a closed vessel. The solution of sul- 



* The detection and separation of arsenic, see the following arti- 

 cle, also poisoning by arsenic. 



7* 



