INSECTICIDES AND FUNGICIDES. 7 



with little danger of injury. It is a slow-acting poison but effective; the 

 fineness of particles and light flocculent character insure a high power of 

 suspension and uniform distribution; the white mixture readily indicates 

 the leaf surface covered, and dries to a film which adheres with great 

 persistence. 



(2) Calcium Arsenate. 



Arsenate of lime was employed as an insecticide about 1912, or possibly 

 earlier. Dry arsenate of lime appears to have been first prepared com- 

 mercially by Riches, Piver & Co. of New York. The late war, with re- 

 sulting high prices, brought the product into prominence. 



As with lead arsenate there are three separate products to be considered, 

 as follows : — 



1. Acid calcium arsenate, CaHAs04H20. 



2. Neutral calcium arsenate, Ca3(As04)22H20. 



3. Basic calcium arsenate, a product of rather variable composition, probably 

 depending on the amount of excess lime. This is the commercial article sold under 

 the name of calcium arsenate. 



Composition . — The composition of theoretical acid and neutral calcium 

 arsenates, and of a commercial basic calcium arsenate, is substantially as 

 follows : — 



Physical Properties. — The calcium arsenates are soft, white powders 

 of fine particles with a good power of suspension and adhesiveness. The 

 specific gravity (15) of a pure acid salt at 20/4° C. was 3.09, and of a 

 neutral salt, 3.23. The commercial dry basic calcium arsenate is a bulky, 

 impalpable powder of 80 to 100 cubic inches to the pound. 



Stability. — The acid salt is largely soluble in water, and the neutral 

 salt appreciably so, as determined by the Hilgard method. Carbonic acid 

 will decompose both salts vdth the formation of carbonate and the libera- 

 tion of arsenic. Dilute solution of alkalies and their salts will increase the 

 solubility of the arsenic, the acid salt invariably proving the more un- 

 stable. I^or these several reasons calcium arsenate used alone burns 

 foliage very badly. As in the case of other arsenicals, milk of lime prevents 

 burning by combining with any soluble arsenic which may be fonned. 



