1912.] PUBLIC DOCUMENT — No. 31. 185 



recorded by Miles ^ and by Fernald - of 3.20 and 3.42 respec- 

 tively, results in a low power of susj)ension as shown by Colby ^ 

 of five minutes for a coarse sample and seventeen minutes for 

 a fine, in 1 foot column of water at the proportion ordinarily 

 applied. Woods and Hanson ■* show as a result of a microscopi- 

 cal examination of 21 commercial samples of Paris green, slow 

 process with possibly one exception, that on the average only 

 5.27 per cent, of the green particles exceeded a diameter of 19.2 

 microns (.00077 of an inch). The ammonia test for purity 

 mentioned by Riley ^ and by Paddock ° is now considered of 

 little value except in determining the presence of insoluble 

 materials such as flour and gypsum added as a filler. 



The presence of free arsenic in Paris green in any appreciable 

 amount is deemed objectionable by all investigators on account 

 of possible injury to the foliage due to its corrosive action. 

 While free arsenic can usually be detected by the microscope, its 

 quantitative determination for a time proved a more difficult 

 matter, and results by the earlier methods were of questionable 

 value except in a comparative sense. Haywood ^ found that 

 Paris green continued to yield arsenic to successive portions of 

 warm water at 50° to 60° C, and also to repeated washings of 

 cold water on a filter. He secured practically constant results 

 bv treating 1 gram of green in a flask with 500 cubic centime- 

 ters of water for twelve days, but subsequent tests ^ showed the 

 presence of soluble copper, indicating either solution or break- 

 ing down of the green. He favored the latter view, but as- 

 sumed that the decomposition was in proportion to original con- 

 tent and corrected the results accordingly. 



Hilgard ® acknowledged that warm water was not permissible 

 and recommended a treatment conforming more nearly to or- 

 chard practice, 1 gram to 1,000 cubic centimeters of cold water 

 for twenty-four hours with prolonged agitation. He questioned 

 any dissociation of the green, but admitted that continued 



1 Va. Exp. Sta., Bui. No. 24, p. 16 (1893). 



2 Mass. Bd. Agr. Kept., 45, p. 3.55 (1898). 



» Oal. Exp. Sta., Bui. No. 15t, p. 34 (1903). 

 * Me. Exp. Sta., Bui. No. 154, p. 114 (1908). 

 6 IT. S. Ent. Com., Bui. No. 3, p. 50 (1880). 

 6 N. Y. Exp. Sta., Bui. No. 121, p. 219 (1897). 

 ' .Tour. Amer. Chem. Soc, 22, p. .579 (1900). 

 '-Tour. Amer. Chem. Soc, 22, p. 705 (1900). 

 » Jour. Amer. Chem. Soc, 22, p. 691 (1900). 



