INSECTICIDES AND FUNGICIDES. 6 



beginning of the era in which active agents supersede the old-time repel- 

 lents (2), which were usually characterized by offensive (both odor and 

 taste) or caustic rather than poisonous properties. Some, however, may 

 have possessed value as contact poisons. In 1875 Scheele's green (3) was 

 introduced, in 1877 London purple (4), and in 1891 calcium arsenite (5). 



There are several other commercial products which are worthy of men- 

 tion, although many of them contain soluble arsenic, and in some cases 

 uncombined white arsenic may have been a constituent. Gray arsenoid 

 was a mixture of calcium and copper arsenites. Barium arsenite or white 

 arsenoid was essentially a mixture of barium compounds and of arsenic 

 rather than a true salt. Zinc arsenite was employed by potato growers in 

 certain localities for a few seasons. Laurel green was a mixture of copper 

 arsenite, gypsum and green sand. Paragrene was a mixture of Paris green 

 and gypsum. 



Paris green, because of its quick action and comparative safeness when 

 applied with lime, still maintains a place in the list of recommended ar- 

 senicals, but has certainly lost much of its former prominence. 



(1) Paris Gh'een. 

 Composition. — Paris green is a copper aceto-arsenite for which the 

 formula of Eugene Ehrmann is generally accepted : — 



(CHsCOO)^ Cu.SCu (As02)2 



As a double salt it may be said to consist of 1 part of copper acetate and 

 3 parts of copper metarsenite. The composition of the theoretical salt 

 is as follows : — 



Per Cent. 

 Cupric oxide (CuO), 31.385 



Arsenous oxide (AsiOs), ..... 

 Equivalent to metallic arsenic (As) (per cent), 

 Ratio CuO : AS2O3, ..... 



Acetic anhydride (CH3CO)20, .... 



58 . 550 

 44.350 

 1 : 1.866 



10.065 



100.000 



Stability. — While Paris green is nominally insoluble in water, it is in 

 reality unstable, and breaks down under continuous percolation of water, 

 with the liberation of soluble and hence injurious arsenic. Carbonic acid, 

 ammonia and certain alkaline salts likewise tend to increase the solubility 

 of the arsenic. Since conditions Avhich control the presence of these sub- 

 stances cannot alwa3'S be foretold, it is alwaj^s well to add milk of lime to 

 Paris green to prevent arsenical injury. It does this by uniting -with the 

 free and loosely combined arsenic to form calcium arsenite, which is in- 

 soluble and hence non-injurious to the foliage. 



Form of Guaranty. — The form of guaranty under which Paris green is 

 sold is as follows : — 



Per Cent. 

 Total arsenous oxide, not less than . . . . . . .50.00 



Water soluble arsenous oxide, not more than . . . . . . 3 . 50 



