ON ARSENIC 121 



acid of arsenic, but it is, when dissolved in vinegar. The 

 above solutions are precipitated by neutral arsenical salts ; 

 the precipitate easily dissolves in acids, but not in water. 

 (c) Treated in the crucible, it shows the same phenomena 

 as the precipitate of lime (Sec. xv. (d)). (d) If the solution 

 (a) be evaporated to dryness, and then mixed with a little 

 charcoal powder, and exposed to distillation with a strong 

 fire, a little yellow sublimate rises into the neck of the 

 retort, as likewise some shining regulus ; volatile sulphureous 

 acid passes over into the receiver. The residuum dissolves 

 with difficulty in vitriolic acid ; some crystals of alum, how- 

 ever, will form in the space of two months. 



SECTION XVIII. UPON WHITE CLAY. 



One part of white clay reduced to powder was mixed 

 with four parts of acid of arsenic, and digested for a 

 fortnight ; but the acid would not dissolve any part of it. 

 The mass was put into a retort, and distilled to dryness, and 

 the fire increased till the retort began to melt. It was con- 

 verted into a thick flux, and a little arsenic was sublimed. 

 The residuum, powdered and mixed with a little charcoal 

 powder, was exposed to distillation in the same degree of 

 heat, when a shining regulus was sublimed. 



SECTION XIX. UPON SILEX. 



Liquor silicum was precipitated by vitriolic acid, and 

 lixiviated with boiling water. While it was still moist, 

 acid of arsenic was added, and the mass was exposed to 

 digestion for fourteen days, but no solution ensued. I 

 afterwards evaporated it, and calcined the product, till the 

 retort which I used began to melt. A little arsenic rose 



