ARSENIC. 179 



flammable. In this form of red phosphorus, it is used 

 in the manufacture of friction matches. 



ARSENIC. 

 438. DESCRIPTION. Arsenic is a grey 



Why is arsen- / . 



ic introduced substance, of metallic lustre, and for this 

 metalloids? reason, commonly classed among the met- 

 als. On the other hand, in view of the 

 compounds which it forms, and especially in view of 

 the fact that its oxygen compounds are acids, and not 

 oxides, it is more properly classed among the metal- 

 loids. Its analogies to phosphorus are most striking, 

 and it is for this reason here introduced, in immediate 

 connection with that element. 



In what re 439. ANALOGIES TO PHOSPHORUS. Ar- 



spccts do pkos- sem ' c unites with oxygen in the same pro- 



phorus and . J ... 



arsenic resem- portions as phosphorus, forming similar 

 acids. These in turn form salts resembling 

 each other most perfectly, in external appearance and 

 in crystalline form. It also combines with three atoms 

 of hydrogen, to form arseniuretted hydrogen, a gas 

 analogous to phosphuretted hydrogen, to be hereafter 

 described. Of the two principal oxygen compounds of 

 phosphorus, the higher, or phosphoric acid, is the more 

 important, and was therefore more particularly consid- 

 ered. On the other hand, the lower orarsenious acid, is 

 the more important of the acids of arsenic. 

 How is arsenic 440. PREPARATION. Metallic arsenic is 

 prepared ? found native. It may also be prepared 



