284 INSECTICIDES AND FUNGICIDES 



216. Insecticides. — These spray materials may be divided 

 into two classes, internal, and external or contact insecti- 

 cides. 



I. INTERNAL INSECTICIDES 



(a) Hellebore is a powder made by grinding the dried 

 roots of the American hellebore (veratrum viride) or of 

 the white or European hellebore (veratrum album). The 

 active constituents are certain alkaloids, said to be some six 

 in number. 



(6) Lead Arsenate is made by mixing sodium arsenate, 

 Na 2 HAs0 4 , with lead acetate, Pb(C 2 H 3 2 ) 2 , or lead nitrate, 

 Pb(N0 3 ) 2 . The precipitate, depending on conditions of 

 temperature, concentration, and methods of mixing, consists 

 of neutral or triplumbic arsenate, Pb 3 (As0 4 ) 2 , and acid lead 

 arsenate, PbHAs0 4 , in varying proportions. It comes on the 

 market as a paste or powder which forms a suspension on 

 mixing with water in the proper proportions for spraying. 

 When eaten by insects the arsenic in the compound is fatal. 



Injury sometimes results to the leaves from the use of 

 lead arsenate. Since insoluble compounds do not harm foliage 

 it is certain that enough arsenic from lead arsenate goes into 

 solution to penetrate the leaves and kill the tissues. Con- 

 sequently the amount of water soluble arsenic in commercial 

 preparations of lead arsenate is limited by the national 

 Insecticide Act of 1910 to 0.75 per cent, of water soluble 

 As^. 1 But injury has resulted from the use of lead arsenate 

 that came well within the law, and as a result of investiga- 

 tion the following facts have been discovered: Triplumbic 

 arsenate or the neutral lead arsenate is insoluble in pure 

 water or water containing chlorides, sulphates, or carbon- 

 ates, and causes no harm to foliage. The acid arsenate, on 

 the other hand, is slightly soluble in pure water and much 

 more so in water containing the above named substances 

 in solution. Injury results when hard water, or "alkali" 



1 Determined by soaking 1 gram of lead arsenate in 1000 c.c. of carbon 

 dioxide-free water for ten days, shaking eight times a day, and analyzing 

 the filtrate. Water soluble AS2O3 in Paris green is determined in the same 

 way. 



