120 The Spraying of Plants. 



goes on slowly, requiring more than twenty-four hours to reach 

 completion." This precipitate is the arsenite of lime, which is 

 the active principle of London purple. 



In 1875 McMurtrie, then the chemist at the Agricultural 

 Department in Washington, conducted some experiments from 

 which he drew the following conclusion : " Plants have not the 

 power to absorb and assimilate from the soil compounds of 

 arsenic, and that though arsenical compounds exert an in- 

 jurious influence upon vegetation, yet this is without effect 

 until the quantity present reaches for Paris green about 900 

 pounds per acre ; for arsenite of potassa about 400 pounds per 

 acre ; for arseniate of potassa about 150 pounds per acre." l 



ARSENATE OF LEAD; GYPSINE; PB 3 (AsO 4 ).,. Arsenate of 

 lead may be made by placing "11 ounces of acetate of lead and 

 4 ounces of arsenate of soda into a hogshead containing 150 

 gallons of water. These substances quickly dissolve and form 

 arsenate of lead, a fine white powder which remains in suspen- 

 sion in water." 2 If it is desired to make any variations in the 

 above, the poison can be prepared by using 29.93 per cent by 

 weight of arsenate of soda, and 70.07 of acetate of lead. These 

 may be dissolved separately, and when united the arsenate of 

 lead will be precipitated. This compound is much lighter than 

 Paris green and can be used with greater freedom, as it does 

 not injure foliage. The conclusions of Fernald in regard to 

 the amounts to use are that "some such proportions as 1, 1|, 

 or 2 pounds to 150 gallons of water would prove entirely satis- 

 factory," 3 and potato beetles were killed when but f of a pound 

 was used. At the Cornell Station this poison proved unsatis- 

 factory in the destruction of canker-worms 4 and of tent-cater- 

 pillars in 1895. 



ARSENATE OF SODA ; NA 2 HAsO 4 . This material has also 

 been tried by Fernald, but it injured foliage and was not so 

 effectual in destroying insects as the other forms commonly 

 recommended. Its use for this purpose cannot be advised. 5 



ARSENITE OF COPPER ; SCHEELE'S GREEN ; CuHAsOo, or 



1 Ann. Rept. U. 8. Com. of Agric. 1875, 14T. 



2 Fernald, Mass. Hatch Agric. Exp. Sta. 1894, April, Bull. 24, 6. 



3 Ibid. 5. 



* Cornell Agric. Exp. Sta. 1895, Bull. 101. 



5 Mass. Hatch Agric. Exp. Sta. 1894, April, Bull. 24, 8, 9. 



