COPPER ACETO-ARSENITE. 271 



it is less injurious when very finely divided. Whitehead gives as harm- 

 less doses, 40 grammes iu 100 litres of water for apple-trees, 45 

 grammes in 100 litres of water for pear-trees and gooseberry bushes, 

 and 60 grammes in 100 litres of water for plum-trees and currant 

 bushes. (For grammes per 100 litres read also parts by weight per 

 100,000 parts by measure.) 



Peach-trees are the most sensitive to copper arsenites ; that is not 

 surprising, since other and even insoluble copper salts, even in minimum 

 doses, induce the total or partial fall of the leaves of that tree. 



Action of Arsenites of Copper on Insects. — The arsenite and 

 aceto-arsenite of copper are violent poisons to all insects, which in 

 nibbling the leaves absorb a small quantity. 



Use. — Arsenical insecticides have been in use since 1859. In 1872 

 Dr. Le Baron recommended the use of Paris green against the cater- 

 pillar of the citigrade spider. In 1878 Haynes tried this product 

 against the pyralis of the apple-tree, and this trial was followed by 

 those of Cook, Forbes, Alvood, and others. In 1887 the use of 

 arsenical salts was recognized as the most efficient treatment for 

 phytophagous insects. In 1896, 2000 tons of emerald green were 

 consumed iu America and 400 tons in Canada. Arsenical sprayings 

 now rarely consist of arsenites alone ; they are mixed with substances 

 which facilitate their application by keeping the arsenites in suspension 

 in water or impart to them anticryptogamic properties. The substances 

 with which they are mixed should be of such a nature as not to render 

 the arsenites soluble ; thus ammonia eau celeste, pure blue vitriol, soap, 

 and ferric chloride are excluded. A mixture highly valued in America 

 is that of emerald green and bouillie bordelaise. Coquillet advises to 

 increase the adherence by the addition of resins rendered soluble by 

 soda. For the same purpose 2 per cent of molasses may be incorpor- 

 ated or 1 per cent of farina (potato starch). This latter substance has 

 also the advantage of maintaining the precipitate of arsenite of copper 

 longer in suspension in the liquid. Petrol presents no advantage. 



Method of Use. — 1. In the Dry State. — Insoluble arsenites are 

 used in the dry state as powder when the want of water renders 

 spraying the arsenites as bouillies impracticable. To prepare an in- 

 secticidal powder 1 lb. of Scheele's green is mixed with 100 lb. of 

 gypsum, 100 lb. of slaked lime, or 67 lb. of farina, or with mixtures of 

 50 lb. of gypsum and 50 lb. of farina, or with 67 lb. of farina and 33 

 lb. of gypsum. Dry treatment can only be applied to small-sized 

 plants owing to the danger to the operator. It is very much used in 

 America against the insects of the cotton plant, and various altises, 

 cabbage caterpillars, etc. The insecticide is not so evenly distributed 

 in the form of powder as it is as bouillies ; it is done wath bellows, 

 hand, or sieve. The operator spreads it whilst walking backwards 

 against the wind. Calm weather is chosen early in the morning when 

 the plants are covered with dew or moistened by rain. 



2. As Bouillies. — Arsenites of copper are used as bouillies chiefly 

 in arboriculture. It is preferable to precipitate the arsenite of copper 

 on the spot by mixing arsenite of soda and blue vitriol with the addi- 

 tion of a little lime rather than make commercial Scheele's green or 



