Insecticides and Related Substances. 299 



to the air, this may be partly oxidized to calcium arsenate. This 

 insecticide carries some impurities brought over from the dye- 

 making process, and as a result of insufficient addition of lime 

 or incomplete boiling some of the arsenious acid may be present 

 in free condition. Haywood examined four samples with the 

 following results: 



Per cent 



Moisture 1 . 87-4 .07 



Sand 2.46-3-55 



Arsenious acid, total 6.40-17.31 



Arsenic acid, total 26.50-35.62 



Arsenious acid, soluble in cold water. . . 1 .44-13.49 



Arsenic acid, soluble in cold water 7.12-19.56 



Lime 23.59-25.09 



Water decomposes both calcium arsenate and calcium arsenite 

 to some extent and consequently a solubility determination with 

 water does not show how much arsenious acid was actually free. 

 These soluble arsenic salts are probably less objectionable than 

 free arsenious acid, although it is recognized that London purple 

 is more injurious to foliage than is Paris green and common 

 arsenic (arsenious oxide) is more harmful than either. This con- 

 dition may be corrected by adding lime to the London purple 

 when suspending it in water for application to foliage. Since 

 it is subject to considerable variation in composition this insec- 

 ticide should be bought on guarantee of purity. 



Calcium arsenite was proposed by Kilgore as an insecticide, 

 following his observations with Paris green. This can be made 

 by boiling one pound of arsenious oxide and two pounds of lime 

 in water and diluting for use. Since this compound has been 

 shown to form about 75 per cent of London purple, it is probably 

 more economical to use the latter insecticide. 



Arsenite of soda is prepared by boiling arsenious oxide with 

 four times its weight of sodium carbonate. The injurious effects 

 of this compound upon potato foliage have been referred to. 

 Similar results were produced in trials of sodium arsenate against 

 the gypsy moth in Massachusetts. 



