CHAP. XI. THE CONTROL OF INSECT PESTS OF CROPS. 99 



equivalent to more than 075 per cent, of Arsenic Pentoxide. 

 The paste is generally used at a strength of 4 to 8 pounds to 

 100 gallons (roughlj one-half to one ounce per gallon), the powder 

 at 2 to 6 pounds to 100 gallons of water. If not obtainable din 

 it may be prepared from Lead Acetate (Sugar ol Lead) and 

 Sodium Arsenate, 44 ounces of the former being dissolved in four 

 gallons of water in a wooden or earthenware container, and sixteen 

 ounces of Sodium Arsenate being dissolved separately in two 

 gallons of water; these two solutions on mixing will provide Lead 

 ate sufficient for dilution to 100 gallons. 



The Arsenates of Lead and other metals are about twii 

 poisonous as the corresponding Arsenates, owing to their greater 

 solubility in water and the gastric juices of insects and other 

 animals. For spraying, therefore, as soluble a form of Arsenic is 

 required as can be safely applied to plants without burning them. 

 Sodium Arsenite cannot be so used as it injure ind hence 



is often applied as a weed-killer. Lead Arsenite may be prepared 

 by dissolving separately in water six ounces of Sodium Arsenite 

 (which is itself prepared by boiling together White Arsenic and 

 Caustic Soda) and thirty-two ounces of L< ■ ; these solu- 



are mixed and made up to 62% gallons tor spraying. But, on 

 account of the extra solubility of the Arsenite. it is better to use 

 Lead Arsenate. 



Zinc Arsenite has been used to some extent in America to replace 

 Lead Arsenate, and is claimed to be more toxic, or at least more 

 rapidly poisonous to insects, one pound of Zinc Arsenite being 

 taken as of equivalent toxicity to three pounds of Lead Arsenate. 

 Zinc Arsenate has also been used as an insecticidal spray at the 

 rate of about four pounds to 100 gallons of water. As the action of 

 Zinc salts on animals usually induces vomiting it is probable 

 that Zinc compounds may find a special use in preference to 

 Lead salts in the particular circumstances found in India where it 

 is especiallj desirable to avoid the poisoning of cattle which may 

 accidentally feed on sprayed plants. 



Paris Green, usually soldasan emerald-green powder, is a double 



acetate and arsenite of copper and maybe called Copper Aceto- 



nite, although its actual 1 omposition is not fixed. It is a violent 



poison and may be applied dry but is betl a liquid spray ; 



t should be mixed with Lime in order to neutralize 

 any soluble arsenii which may be present and which would burn 

 the foliage. When applied dry. three to four times the same 

 weight of air-slaked Lime is , id, led and the mixture dusted lightly 

 over the plants in the early morning bj I a muslin 



7 -A 



