ARSENATE OF LEAD. 67 



of lead acetate solution. A white precipitate indicates the 

 presence of soluble arsenic. From the injury to foliage pro- 

 duced by soluble arsenic in spraying mixtures, it is of prime 

 importance that the liquid should show the presence of an 

 excess of lead. When the nitrate of lead is used, the reaction 

 may sometimes show an excess of lead before the arsenic is 

 fully precipitated. In such cases, if the test for arsenic be 

 made, a decided reaction will reveal its presence, and it 

 becomes necessary to add a sufficient quantity of lead nitrate 

 to complete the reaction. 



The gypsy moth committee use annually several tons of 

 arsenate of lead, preparing it, previous to 1897, according to 

 the formula originally given by Mr. Moulton, " sodic arse- 

 niate, 29.93 per cent. ; plumbic acetate, 70.07 per cent." 

 (Report of the Massachusetts State Board of Agriculture, 

 1893, p. 282.) Careful investigations have shown that there 

 is a difference in the chemical composition of these com- 

 mercial salts that cannot be ignored, and that single arsenic 

 and lead salts cannot be taken as types to fix the relative 

 proportions. These salts were mixed together, weighed out 

 in suitable quantities and placed in bags. Previous to being 

 added to the contents of the spraying tank, this mixture was 

 boiled in a kettle until the reaction between the salts was 

 complete. While results obtained from arseuate of lead 

 prepared in this manner are far superior to those obtained 

 by the use of Paris green, several objectionable features 

 developed, the principal one being an incomplete reaction 

 between the mixed salts stored in bags. By this process the 

 particles of arsenate of soda became coated with a thin crust 

 of arsenate of lead, and when sprayed upon the foliage broke 

 down, liberating soluble arsenic and injuring the leaves. 

 Arsenate of lead mixture prepared by grinding together the 

 commercial salts is also of variable composition, and gives 

 very uneven results. Some samples contain a great excess 

 of lead, while in others the arsenate of soda predominates. 

 Because of the incomplete reaction previously mentioned and 

 the uneven composition of the mixture, the arsenate of lead 

 thus obtained often possesses mechanical properties that 

 hinder its application to the foliage. 



The best results in the preparation of arsenate of lead have 



