EXPERIMENTS WITH INSECTICIDES UPON POTATOES. 175 



have carried as high as 68 per cent. In the case of the goods 

 with the low percentage, the relatively small amount of arsenic is 

 due to impurities of the materials. In the case of green carrying 

 more than 58.^ per cent of arsenious acid, the higher arsenic con- 

 tent can only be explained by their having an excess of uncom- 

 bined arsenious acid (white arsenic). As white arsenic 

 burns foliage much more than does Paris green, or even 

 London purple, its presence in Paris green is objection- 

 able. The purity of a Paris green is not necessarily indicated 

 by its arsenic content since an excess of uncombined white arsenic 

 is nearly as dangerous an adulterant as the presence of inert 

 foreign matter. When pure, Paris green should have at least 

 50 per cent of arsenious oxide and should be practically free from 

 uncombined arsenic. Important as the purity of the green is, its 

 mechanical condition is of great moment. To thoroughly pro- 

 tect the plant it is necessary that the poison be thoroughly dis- 

 tributed. It follows therefore that of two equally pure greens, 

 the one that is in the finer powder will prove the more effective. 

 In our experience there is greater danger of purchasing imper- 

 fectly pulverized, than adulterated Paris green. 



The purity of Paris green can be quite readily and fairly 

 accurately tested by dissolving the Paris green in strong ammo- 

 nia water. If pure all of the Paris green will dissolve, the solu- 

 tion turning a deep blue color. Undissolved sediment indicates 

 impurities or adulteration. Another test is to place a little of the 

 Paris green between two pieces of window glass and rub them 

 together. If the Paris green is adulterated with lime, barium 

 sulphate, or similar white materials, the Paris green will appear 

 to turn white in places. Paris green of good quality is intensely 

 bright green and uniform. When adulterated, the green loses 

 something of its intensity and is grayish green and is not always 

 uniform. 



In the experiments here reported upon, Paris green was used 

 in connection with some form of Bordeaux mixture in all of the 

 check plots. 



The Paris green was applied at the rate of one-half pound to 

 the acre. The first application was made before any of the eggs 

 had hatched and may have been unnecessary. The three appli- 

 cations of Paris green at the rate of one-half pound to the acre 



