190 



METALS AND THEIR COMBINATIONS. 



oxide or 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 



FIG 



(Illllllllllllllill 



iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii 



Marsh's apparatus for detection of arsenic. 



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 arsenious oxide ; a cold piece of porcelain held in 

 the flame becomes coated with a brown stain (arsenic spot) of 

 metallic arsenic. (See explanation above in connection with 

 arseniuretted 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. 



The only element which, under the same conditions, forms 

 spots and mirrors similar to arsenic, is antimony; there are, 

 however, sufficiently reliable tests to distinguish arsenic spots 

 from those of antimony. 



Arsenic spots treated with solution of hypochlorites (solution 

 of bleaching-powder) dissolve readily ; antimony spots are not 

 affected. When nitric acid is added to an arsenic spot, evapo- 

 rated to dryness and moistened with a drop of silver nitrate, it 



