INSECTICIDES AND FUNGICIDES. 7 



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

 fineness of i)articles and light floceulent 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) Colcium Arsenate. 



Ai-senate 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 w^ar, 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, CaHAsOjH^O. 



2. Neutral calcium arsenate, Ca3(As04)22H:0. 



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. 



Covi'position. — The composition of theoretical acid and neutral calcium 

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

 follows : — 



Acid Calcium 



Arsenate 



(Per Cent). 



Neutral Calcium 



Arsenate 



(Per Cent). 



Commercial 



Basic Calcium 



Arsenate 



(Per Cent). 



Calcium o.^cle (CaO), .... 



Arsenic pentoxide (As20i), 



Equivalent to metallic arsenic (As) 



(per cent). 

 Ratio AsiOs: CaO 



Water of combination 



37.848 

 1 : 0.488 



28 310 

 58.045 



13.645 



34.531 

 1 : 0.732 



38.744 



52.957 



.299 



44.128 

 45.238 



29.497 

 1 : 0.975 



100.000 



100.000 



10.634 

 100.000 



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 coimnercial dry basic calcium arsenate is a bulky, 

 impalpable powder of SO 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 ^^■ith 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. For 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 ^\ith anv soluble arsenic which mav be formed. 



