ARSENIC. 353 



formed. All these precipitates are soluble in ammonia water and in 

 acids. 



4. Add to a little of the arsenate solution, a clear mixture of mag- 

 nesium sulphate, ammonium chloride and ammonia water, and shake ; 

 a white precipitate of ammonium magnesium arsenate is formed, 

 NH 4 MgAsO 4 (see test 1 under phosphoric acid). 



Magnesium arsenite is insoluble in water, but soluble in ammonia 

 water and in ammonium chloride solution. 



5. Add to a few drops of the arsenate solution, excess of ammonium 

 molybdate solution (about 5 c.c.) and warm gently ; a* yellow precipi- 

 tate of ammonium arseno-molybdate is formed, similar in all respects 

 to the corresponding phosphorus compound. (Arsenic is the only 

 other element which behaves like phosphorus toward the molybdate 

 reagent.) Arsenites give no precipitate with the reagent. 



6. Heat any dry arsenic compound, after being mixed with some 

 charcoal and dry potassium carbonate in a very narrow test-tube (or, 

 better, in a drawn-out glass tube having a small bulb on the end) : the 

 arsenic compound is decomposed and the element arsenic deposited 

 as a metallic ring in the upper part of the contraction. (Fig. 44.) 



FIG. 44. 



7. Heat arsenous or arsenic oxide upon a piece of charcoal by 

 means of a blow-pipe : a characteristic odor of garlic is percep- 

 tible. 



8. Reimch's test. A thin piece of copper, having a bright metallic 

 surface, placed in a solution of arsenic, strongly acidified with con- 

 centrated hydrochloric acid, becomes, upon heating the solution, coated 



23 



