152 AGRICULTURAL CHEMISTRY 



201. Occurrence of Arsenic. This element occurs in 

 the free state to a limited extent, but usually in combina- 

 tion with other elements, as oxygen, iron, and sulfur. 

 In some of its properties, arsenic resembles phosphorus, 

 and forms similar compounds, although arsenic has 

 weaker acid properties than phosphorus. It forms a large 

 number of compounds, among which are the arsenates, 

 and arsenites which are salts of arsenic and arsenious 

 acids. In the presence of a strong base element, arsenic 

 deports itself as an acid while, in the presence of a strong 

 acid element, it exhibits basic properties. Other ele- 

 ments, particularly antimony and bismuth, and to a less 

 extent aluminum, have this same property of acting both 

 as an acid- and base- forming element. Some of the com- 

 pounds of arsenic are extensively used as pigments and 

 insecticides. 



202. Paris Green. Pare Paris green is an aceto-arse- 

 nite of copper and has the following composition : Cop- 

 per oxid, 31.29 per cent., arsenious oxid, 58.65 per cent., 

 acetic acid, 10.06 per cent. Some of the commercial 

 grades of Paris green contain soluble forms of arsenic, 

 while others are adulterated with lime and insoluble sili- 

 cates. The arsenic should be insoluble and have 

 no injurious effect upon vegetation. In case soluble 

 arsenic is present, the foliage is destroyed. Pure Paris 

 green should completely dissolve in hydrochloric acid. 

 In case silica is present, an insoluble residue appears 

 when the material is treated with hydrochloric acid. 

 London purple and various arsenates and arsenites are 

 occasionally used for insecticides. London purple con- 



