ARSENIC. 



213 



FIG. 16. 



chloride (not as sulphide), is poured into the flask through the funnel- 

 tube. If arsenic is present in not too small quantities, the gas ignited 

 at the end of the glass tube shows a flame decidedly different from 

 that of burning hydrogen. The flame becomes larger, assumes a 

 bluish tint, and emits an odor of garlic, while above it a white cloud 

 appears which is more or less dense ; a cold test-tube held inverted 

 over the flame will be covered upon its walls with a white deposit of 

 minute octahedral crystals of arse nous oxide; a piece of cold porce- 

 lain held in the flame becomes coated with a brown stain (arsenic 

 spot) of metallic arsenic. (See explanation above in connection with 

 arsenetted hydrogen.) 



The glass tube heated, as above mentioned, at one of its wide parts, 

 will show a bluish-black metallic mirror at the constriction beyond. 



If quantitative determination is desired, the glass tube is heated in 

 two places so as to cause all hydrogen arsenide to be decomposed. To 

 collect, however, the arsenic from any gas that might escape, the end 

 of the tube is inverted and placed into solution of nitrate of silver, 

 which is decomposed by the hydrogen arsenide, silver and arsenous 

 acid being formed. The arsenic solution should be 

 introduced into the hydrogen generator in small por- 

 tions, so as to produce not more hydrogen arsenide 

 at a time than can be decomposed by the method 

 given. 



The only element which, tinder the same condi- 

 tions, forms spots and mirrors similar to arsenic, is 

 antimony; there are, however, sufficiently reliable 

 tests to distinguish arsenic spots from those of anti- 

 mony. 



Arsenic spots treated with solution of hypochlorites 

 (solution of bleaching-powder) dissolve readily ; anti- 

 mony spots are not affected. When nitric acid is 

 added to an arsenic spot, evaporated to dryness and 

 moistened with a drop of silver nitrate, it turns 

 brick-red; antimony spots treated in like manner 

 remain white. Arsenic spots dissolved in ammonium 

 sulphide and evaporated to dryness show a bright yellow, antimony 

 spots an orange-red residue. 



Fig 16 represents a simpler form of Marsh's apparatus, which 

 generally will answer for students' tests. 



Preparatory treatment of organic matter for arsenic analysis. If 

 organic matter is to be examined for arsenic (or for any other metallic poison) 



Student's appa- 

 ratus for making 

 arsenic spots. 



