No. C. DEPARTMENT OF AGUICULTUKE. 611 



no doubt, of so-called improvements in the method of maniifticture, 

 leading to an increased product at less expense. Pari» green when 

 pure varies both in composition and the proportions of its chemical 

 ingredients. Add to this the variability brought about in its manu- 

 facture, and it will readily be seen how exceedingly variable and 

 unsatisfactory a product will result. 



Investigation has shown that most of the injury caused by Paris 

 green is due to an excessive proportion of free arsenious oxid 

 (white arsenic), either remaining as the result of careless manu- 

 facture or wilfully put in to bring up a product low in arsenic to a 

 standard strength. Free arsenious oxid is soluble in water after a 

 time, and when it is present in Paris green to any great extent, de- 

 stroys one of the latter's most valuable qualities as an insecticide, 

 its great insolubility in water. It is this latter quality which makes 

 the use of Paris green possible without injury to the foliage. Free 

 arsenious oxid is at times .extremely injurious to the foliage, especi- 

 ally when in solution. This seems especially true in dry localities, or 

 during dry weather having hot days followed by heavy dewis or fogs 

 at night. The arsenic seems to be more soluble under these condi- 

 tions, and is dissolved bv the dew and absorbed bv the leaves in suf- 

 ficient quantitiy to cause injury. More investigations of the subject 

 are necessary, however, for at times it has been possible to use pure 

 solutions of white arsenic without injury. But until it is known 

 more definitely under what conditions the solutions may be used, it 

 is safer to stick to the insoluble material. 



The other complaint entered against Paris green, its ineffective- 

 ness, was found to be the result of a reduced proportion of arsenic, 

 the active poisonous principle of the material. It requires a certain 

 amount of poison to kill an insect. Naturally, then, if a weaker 

 green is applied, an amount of arsenic sufficient to cause the insect's 

 death may not be present. This was really the first defect found in 

 the manufactured poison, and led to legislation in some States — 

 notably New York — stipulating that Paris green offered for sale 

 shall consist of not less than fifty per cent, of arsenious oxid. This 

 may have been responsible, to some extent at least, for the other 

 count against the poison, its excessive proportion of free arsenious 

 oxid, by leading manufacturers "to fill" a green low in combined ar- 

 senic, with free white arsenic. Such legislation, therefore, reached 

 only half way. The law has lately been amended so as to state defi- 

 nitely, not only the total percentage of arsenic a sample should con- 

 tain, but also the limit of arsenious oxid uncombined with copper. 

 In California the limit of free arsenious oxid has been found to be 

 four per cent., and that limit has been adopted in the law of that 

 State.* 



*(BaUetin 126, CaUfornia]ExperimeQt'Station.) 



