476 FRUIT INSECTS 



variable, the arsenic content varying from 30 to 50 per cent. 

 It is a finer powder than Paris green and, therefore, remains 

 longer in suspension in water. It is used in the same way as 

 Paris green, but owing to the presence of much soluble arsenic 

 is likely to cause foliage injury. This can be averted by the 

 use of lime as advised under Paris green. London purple is 

 now little used in orchard spraying. 



Arsenate of Lead 



Arsenate of lead was first used as an insecticide in 1893, 

 by the Gypsy Moth Commission of Massachusetts, as a sub- 

 stitute for Paris green, since it had been found that the latter 

 poison would seriously injure the foliage if applied suflftciently 

 strong to kill the gypsy moth caterpillars. It has now almost 

 entirelj^ replaced Paris green and London purple for orchard 

 work throughout the country. It adheres better to the leaves, 

 may be used at considerably greater strength without injuring 

 the foliage and may be combined with the dilute lime-sulfur 

 solution when used as a summer spray. Combined with the 

 self-boiled lime-sulfur it can be safely used on the peach. 



Chemically, arsenate of lead may be either triplumbic ar- 

 senate, Pbs (As04)2, or plumbic hydrogen arsenate, PbHAs04. 

 The commercial product usually consists or a mixture of these 

 two forms, the proportion depending on the method of manu- 

 facture employed. The triplumbic arsenate of lead is prepared 

 by combining normal sodium arsenate (Na3As04) with either 

 lead acetate (PbC2H302) or lead nitrate (Pb (N03)2). If any 

 di-sodium hydrogen arsenate (Na2HAs04) be present, there 

 is then formed some of the plumbic hydrogen arsenate. 



Arsenate of lead is usually sold in the form of a thick paste, 

 but for some purposes the powdered form is preferred. Under 

 the National Insecticide Act of 1910, arsenate of lead paste 

 Uiust not contain more than 50 per cent water and must con- 



