224 ELEMENTARY AGRICULTURAL CHEMISTRY 



some risk of poisoning, for arsenic is not a substance which is 

 readily eliminated from the carcass by decomposition, as some 

 poisonous substances are. 



The arsenite of soda employed for this and other purposes 

 may be prepared by boiling " white arsenic " — i.e., arsenious 

 oxide — with one-third of its weight of caustic soda or four times 

 its weight of sodium carbonate (washing soda) and water until 

 it dissolves ; or it may more conveniently be procured already 

 prepared in the form of a white solid. 



About 9 ounces of " white arsenic " is equivalent to 1 lb. 

 of sodium arsenite. 



The sugar in the above formula is intended to make the 

 poisoned material more attractive to the insects, and aids also 

 in increasing the quantity which adheres to the grass or other 

 vegetation. 



Arsenic is also used for the destruction of caterpillars, 

 grubs, &c., particularly on fruit-trees. In this case the use of 

 arsenious oxide, arsenite of soda, or other readily soluble com- 

 pound is excluded, because of the injury which such substances 

 produce on the foliage. 



Several almost insoluble compounds of arsenic are therefore 

 employed, the following being the favourites : 



PaiHs Green, " Schweinfurth's Green," or " Emerald Green." — 

 An impure arsenite and acetate of copper, usually containing 

 from 30 to 50 per cent, of arsenious oxide (in combination), 

 but of very variable composition. Often a portion (2 per cent. 

 or more) of its arsenic is in a soluble form. The best samples 

 for spraying are those which contain the least soluble and the 

 most insoluble arsenic. It is used in suspension in water, and 

 applied by means of a spray pump, usually at a strength of 

 1 part of the solid in 2000 or 3000 of water. Obviously, to 

 ensure good results the liquid must be kept in constant agita- 

 tion, otherwise the Paris green will settle to the bottom. The 

 injury to foliage due to the presence of soluble arsenic may be 

 prevented by the addition of an equal weight of lime. 



London Purple. — A mixture of arsenite of lime with colour- 



