INSECTS AND INSECTICIDES 371 



and, therefore, does not have as great a tendency to burn the 

 foliage; it adheres better to the leaves but is a little slower 

 in its action on the insects. 



Chemically, arsenate of lead may be either triplumbic arse- 

 nate, Pb3(As04)2, or plumbic hydrogen arsenate, PbHAs04. 

 The commercial product usually consists of 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, wdth either 

 lead acetate, Pb (0211302)2, 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. Arse- 

 nate of lead is sold in two forms, a thick paste and a fine powder. 



Under the National Insecticide Act of 1910, arsenate of lead 

 paste must not contain more than 50 per cent water and must 

 contain the arsenic equivalent of at least 12i per cent arsenic 

 oxid, AS2O5. The water-soluble arsenic must not exceed an 

 equivalent of f of one per cent of arsenic oxid. Some of the 

 commercial preparations contain a larger percentage of arsenic 

 than required by the law. In the best grades of arsenate of 

 lead paste, the chemical is in a finely divided condition, and 

 thus when diluted for use remains in suspension for a consider- 

 able time. If the paste contains less than 50 per cent water, 

 it is likely to be lumpy and requires considerable time and labor 

 to get it into condition for use. 



Powdered arsenate of lead is usually considered to contain 

 twice as much arsenic as the paste. It is often stated on theoret- 

 ical grounds that powdered arsenate of lead does not adhere 

 as well to the foliage as the paste form, but practically there is 

 little if any difference. The powder is more convenient to use, 

 it mixes more readily with water and broken packages are not 

 injured by drying out or by freezing. In the preceding chapters 

 the quantity of arsenate of lead to be used has been given in 

 terms of the paste form but in most cases the powdered form 



