PHOSPHORUS, ARSENIC, ANTIMONY, BISMUTH 323 



coal, and magnesium powder, and heated in a narrow tube closed 

 at one end, give a phosphide of the metal (Na 3 P or Mg 3 P 2 ) . When 

 the mass is moistened, the odor of phosphine PH 3 can be recog- 

 nized: 



Na 3 P + 3H 2 -* 3NaOH + PH 3 1 . 



Shells containing metallic phosphides are of service in naval 

 warfare as markers in night engagements, the impure phosphine 

 produced by the action of the water on the phosphides burning 

 spontaneously on the surface and indicating the position of the 



OLAJ. i^ <WH*^ M^^<^^ f^^ 



Phosphorus and Nitrogen Compared. Although tfie 

 simple substances, phosphorus and nitrogen, have little in com- 

 mon, they form compounds of similar composition, which are in 

 many ways alike. Thus we have ammonia NH 3 and phosphine 

 PH 3 , both gases. The phosphides are analogous to the nitrides 

 (p. 287). Then there are the oxides N 2 O 3 and N 2 5 , to which the 

 oxides P 2 3 and P 2 5 correspond. All these oxides are anhydrides 

 of acids. Both nitrogen and phosphorus are typically non- 

 metallic elements (p. 274), entering into negative radicals. Both 

 elements are trivalent in one series of compounds (NH 3 , N 2 O 3 , 

 PH 3 , P 2 3 ) and quinquivalent (N 2 O 5 , P 2 5 ) in another series. 



ARSENIC As 



Arsenic, the third member of this family, is metallic in appear- 

 ance and in physical properties, although in combination it behaves 

 as a non-metallic element. The metal is used with lead in making 

 small shot, and the oxide As 2 3 in preparing medicines and insec- 

 ticides. 



Preparation. Arsenical pyrites FeSAs, a mineral similar to 

 pyrite FeS 2 , but containing arsenic in place of half of the sulphur, 

 is one of the commonest natural forms of arsenic. When this 



