In the preparation of the arsenate of lead it is only necessary to form a 

 chemical union between the common lead oxide (litharge), PbO, and arsenic 

 pentoxide, (As2Os), but in order to obtain a product suitable for use as an 

 insecticide, the chemcial union must take place between soluble salts contain- 

 ing these oxides. In general practice, arsenate of lead suitable for spraying 

 purposes is prepared by bringing together commercial grades of acetate or 

 nitrate of lead and arsenate of soda. Owing to the variable composition of 

 these commercial salts, a chemical analysis is indespensible, as indicating the 

 relative amounts to be used. All such calculations must be based on the 

 quantity of lead oxide (PbO) found in the lead salt and that of arsenic pen- 

 toxide (As2 O5 ) contained in the arsenate of soda, making due allowance for 

 the other acidulous radicals which may precipitate the lead. 



Where the acetate of lead is used approximately the whole of the arsenate 

 of lead product consists of tri-plumbic arsenate, as indicated in the following 

 equation. 



3Pb(C2 H3 O2 ) 2; 3H2 O+2Na2 H AsO4 (H2O)n- 

 Pb3 (AsO4)2 + 4NaC2H3O2 + 2C2 H4 O 2 + nHzO. 



The following equation represents the reaction between arsenate of soda and 

 nitrate of lead by Smith:--(l) 



5Pb (No 3 )2 +4N a 2 HAs0 4 (H 2 0)n = 



Pb3(AsO4 )2 + 2PbHAsO4 + 8NaNO 3 + 2HNO3 + n (H2 O). 



EFFICIENCY AS A SPRAYING MATERIAL. 



Arsenate of lead remains in suspension in water much longer than Paris 

 green, because of its very low specific gravity, which is 1,00688 while that of 

 Paris green is 3,42225. In spraying, the low specific gravity of arsenate of lead 

 and its consequent suspension in water for a considerable length of time make 

 it possible to distribute it more evenly over vegetation. The white color is also 

 a decided advantage, for one is enabled to see at a glance whether a tree or 

 shrub has been sprayed; and it is a noteworthy fact that this insecticide adheres 

 to foliage for longer than any similar substance now in use. It is undoubtedly 

 true that larger proportions of arsenate of lead must be used than of Paris 

 green but this can be done with entire safety to the vegetation. The cost of 

 the insecticide forms a very small part of the cost of spraying; and since arsen- 

 ate of lead remains on the foliage so much longer than other insecticides, a 

 much larger proportion can be used and even then be much cheaper than 

 substances which wash off readily in showers, making it necessary to spray 

 the trees the second time. 



A large percentage of the spraying at the present time is with a mixture 

 of an insecticide and a fungicide; because as has already been said, the great 

 expense is in the labor, and not in the materials used; and when the insecticide 

 and fungicide can be applied together, the cost of one spraying is saved. Ar- 



(1) Annual Report State Board of Agriculture, 1897, page 354. 



