AESENIC. 183 



Pt, Mo) which are classed together in this group. In fact, the 

 only resemblance which unites these metals is the insolubility of 

 their sulphides in diluted acids and the solubility of these sul- 

 phides in ammonium sulphide (or alkaline hydrates), with which 

 they form soluble double compounds ; the oxides have also 

 a tendency to form acids. In all other respects no general 

 resemblance exists between these metals. Arsenic and anti- 

 mony have many properties in common, and resemble in many 

 respects the non-metallic elements phosphorus and nitrogen, as 

 may be shown by a comparison of their hydrides, oxides, acids, 

 and chlorides : 



NH 3 X 2 3 N 2 O 5 NC1 3 . 



PH 3 P 2 3 P 2 5 H 3 P0 4 PC1 3 . 



AsH 3 As 2 O 3 As 2 O 5 H 3 AsO 4 AsCl 3 . 



SbH 3 Sb 2 O 3 Sb 2 O 5 SbCL. 



o & 6 it o 



Arsenic. Found in nature sometimes in the native state, but 

 generally as sulphide or arsenide. One of the most common 

 arsenic ores is the arsenio-sulphide of iron or mispickel, FeSAs. 

 Realgar is the native red sulphide, As 2 S 2 , and orpiment or awri- 

 pigment the native yellow sulphide, As 2 S 3 . Arsenides of cobalt, 

 nickel, and other metals are not unfrequently met with in 

 nature. Certain mineral waters contain traces of .arsenic com- 

 pounds. 



Arsenic may easily be obtained by heating arsenious oxide 

 with charcoal, or by allowing vapors of arsenious oxide to pass 

 over charcoal heated to redness : 



As 2 O 3 + 3C = 3CO + 2As. 



In both cases the arsenic, when liberated by the reducing 

 action of the charcoal, exists in the form of vapors, which con- 

 dense in the cooler part of the apparatus as a steel-gray metallic 

 mass, which, when exposed to the atmospheric air, loses the 

 metallic lustre in consequence of the formation of a film of 

 oxide. 



When pure, arsenic is odorless and tasteless ; it is very brittle 

 and volatilizes unchanged and without melting when heated to 

 180, without access of air. Heated in air, it burns with a 

 bluish- white light, forming arsenious oxide. Although insoluble 

 in water, yet water digested with arsenic soon contains some 



