LESSONS IN CHEMISTRY. 145 



As 2 5 , combined with water, forms arsenic acid, which forms 

 arseniates. Arsenic acid by its action upon aniline forms the 

 beautiful dye called magenta. It is also much used in calico 

 printing. There are three sulphides of arsenic. Kealgar, or red 

 orpiment, formula As 2 S 2 , is a beautiful mineral crystallizing in 

 orange red prisms. It is an important ingredient in compositions 

 used for fire-works and signal lights. Yellow orpiment, As 2 S 3 , 

 is found native as yellow prismatic crystals. A fine yellow paint 

 is made from this substance. The other sulphide is not of special 

 interest. 



Arsine is a compound of arsenic and hydrogen, formula, 

 AsH 3 , analogous to ammonia and phosphine. It is a color- 

 less, poisonous gas, having an unpleasant odor. Arrange an 

 apparatus as in preparing hydrogen, adding a little arsenic. 

 Arrange a delivery tube so that the gas formed may be burned. 

 When the gas is burning freely bring into the flame a piece of 

 porcelain, and a dark spot is formed by the condensed arsenic 

 called a mirror of arsenic. When carefully performed this is 

 a very delicate test for the presence of arsenic, and is exten- 

 sively used in examining the contents of the stomach of per- 

 sons in cases of suspected poisoning. It is known as Marsh's 

 test. Arsenic and its compounds are notable for their poisonous 

 properties, and for their bright and beautiful colors. 

 L. s. 10 



