AESENIC. 205 



30. ARSENIC. 



As =*= 74 9. 



General remarks regarding- the metals of the arsenic group. 

 The metals belonging to either of the five groups considered hereto- 

 fore, show much resemblance to each other in their chemical prop- 

 erties, and consequently in their combinations. This is much less 

 the case among the six metals (As, Sb, Sn, Au, 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 dilute 

 acids and the solubility of these sulphides in ammonium sulphide 

 (or alkali hydroxides), with which they form soluble double com- 

 pounds ; the oxides have also a tendency to form acids. In all other 

 respects no general resemblance exists between these metals. Arsenic 

 and antimony 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 NA NA NCI,. 



PH 3 PA P 2 Qs H 3 P0 4 PC1 3 . 



AsH 3 AsA AsA H 3 AsO 4 AsCl 3 . 



SbH 3 SbA SbA SbCl 3 . 



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 mispielcel, FeSAs. Realgar is 

 the native red sulphide, As 2 S 2 , and orpiment or auripigment, the native 

 yellow sulphide, As 2 S 3 . Arsenides of cobalt, nickel, and other metals 

 are not infrequently met with in nature. Certain mineral waters 

 contain traces of arsenic compounds. 



Arsenic may be obtained easily by heating arsenous oxide with 

 charcoal, or by allowing vapors of arsenous oxide to pass over char- 

 coal heated to redness : 



As 2 O 3 -f 3C = SCO -f 2As. 



In both cases the arsenic, when liberated by the reducing action of 

 the charcoal, exists in the form of vapor, which condenses in the 

 cooler part of the apparatus as a steel-gray metallic mass, which 

 when exposed to the amospheric air, loses the metallic lustre in conse- 

 quence 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 C. 



