No. 4.] GYPSY MOTH — APPENDIX. 357 



ARSEl^ATB OF LEAD: ITS MANUFACT- 

 URE ANT> CHEMICAL COMPOSITION. 



FREDERIC J. SMITH, M.S., CHEMIST TO THE COMMITTEE. 



The value of arsenate of lead as an insecticide has been 

 fully set forth in the preceding paper by the entomologist. 

 Since this insecticide has come into such general use, the 

 present seems a proper time to place on record the essential 

 facts concerning the ingredients used in making arsenate of 

 lead, the chemical reactions that are involved and the exact 

 composition of the final product. The results of our work 

 will, it is hoped, be all the more serviceable to economic 

 entomologists because it is apparent that the preparation 

 of this substance has not been generally understood, as is 

 shown by several erroneous and conflicting statements which 

 have been published concerning this subject. 



To be exact, the arsenate of lead used in spraying oper- 

 ations is not a salt whose composition may be definitely 

 expressed by a single formula, but instead is a mixture of 

 both di-plumbic and tri-plumbic arsenates, the relative quan- 

 tities of each depending principally upon the source of the 

 soluble lead salt used. Since the term "arsenate of lead" 

 has now become so well established, it will be used in its 

 general sense in the present paper. 



Ingredients. 



Theoretically, in the preparation of arsenate of lead it is 

 only necessary to form a chemical union between the com- 

 mon lead oxide (litharge), PbO, and arsenic pentoxide, 

 AsoOg ; but, in order to obtain a product suitable for use as 

 an insecticide, the chemical union must take place between 

 soluble salts containing these oxides. In general practice, 

 arsenate of lead suitable for spraying purposes is prepared 

 by bringing together commercial grades of acetate or nitrate 



