202 EXPERIMENT STATION. [Jan. 



and an electrolyte of sodium nitrate. Patents have also been 

 taken out on various other methods of manufacture, references 

 to which are found in technical journals. I. W. Dvummond 

 patented a dry preparation of lead nitrate, sodium arsenate and 

 corn starch to be mixed with water when applied. 



Most authorities recognize neutral orthoarsenate of lead, Pbs 

 (As04)2, and acid arsenate, PbHAs04, and a few mention 

 pyroarsenate, Pb2As207. W. 11. Volck ^ claims the latter salt 

 may occur in commercial pastes, though Lefevre " states that 

 it is decomposed by cold water. Pyroarsenate differs from 2 

 molecules of the acid salt by 1 molecule of water. 



Pb2As207 + H2O = 2 PbHAs04. 



So far as noted, the presence of pyroarsenate in insecticides 

 has not been proved. 



The low specific gravity of lead arsenate, 1.00G68 according 

 to Smith ^ (salt not specified), results in a high power of sus- 

 pension as shown by Colby ,^ from nitrate one hundred and 

 thirty minutes and from acetate two hundred and forty minutes. 

 Investigators have found lead arsenates comparatively little 

 affected by hot water or carbonic acid. Dilute solutions of 

 sodium carbonate, sodium chloride and sodium sulfate have an 

 appreciable action as shown by Headden ^ and others. The 

 acid salt has invariably proved the more unstable. Volck ^ 

 noted that under alkaline conditions it tends to decompose with 

 the formation of the ortho salt and arsenic acid, and he states 

 that this reaction appears to take place in the orchards of the 

 Pacific coast as a result of the continuous fogs and heavy dews. 

 P. J. O'Gara ^ also claims that the acid salt is very injurious 

 under certain climatic conditions. Haywood ^ recommended 

 the addition of lime to arsenate of lead to prevent injury to 

 delicate foliage. 



'Science, 33, p. 868 (1911). 



2 Cited by A. M. Comey, Diet, of Chem. Sol., p. 35. 



» Mass. Bd. A?r. Kept., 45, p. 355 (1898). 



* Cal. Exp. Sta., Bui. No. 151, p. 34 (1903). 



» Col. Exp. Sta., Bui. No. 131, p. 22 (1908); Bui. No. 157, pp. 29, 30 (1910). 



• Loc. cit. 



'Science, 33, p. 900 (1911). 



« Bur. Chem., Bui. No. 131, p. 49 (1910). 



