ARSENIC. 



an oxide of arsenic, composed of one atom arse- 

 nic and one atom oxygen, and, consequently, 

 weighing 5-^5. When very thin crusts of me- 

 tallic arsenic are exposed to the air, they soon 

 lose their metallic lustre, and crumble into a 

 black powder. This black powder is probably 

 the oxide in question ; but it has not been parti- 

 cularly examined by modern chemists. When I 

 was occupied with the experiments related in this 

 section, I made some attempts to investigate its 

 composition, but they were not successful, and 

 I have not again had leisure to resume them. 



,5. Arsenic has the property, likewise, of com- Arsenietted 



i -,i i J J > j? hydrogen. 



bining with hydrogen gas, and of forming a gase- 

 ous compound, which was first discovered by 

 Scheele, but has never yet been obtained in a 

 state of purity. From our knowledge of the 

 constitution of the other compounds of the com- 

 bustible substances with hydrogen gas, however, 

 there can be no difficulty in stating its leading 

 properties from theory. 



It is probably a compound of one atom arsenic 

 and one atom hydrogen, and, consequently, its 

 atomic weight must be 4*875. 



From the analogy of the other gaseous com- 

 pounds of hydrogen and a combustible, there 

 seems little doubt that it is a compound of 



1 volume arsenic vapour} 



1 volume hydrogen gas Condensed into 1 volume. 



We will obtain its specific gravity in the gaseous 



16 



