53 



a hadly made or very fine Paris green decompose to a great extent in 

 this length of time. It has been found by the author that it takes 

 nearly ten days for all free arsenious oxid in Paris green to go into 

 solution. It has also been found that badly made Paris greens are 

 broken up enough during the course of ten days to show whether they 

 are very unstable or not and that well-made Paris greens are not 

 broken to a great extent in this length of time. Avery and Beans" 

 have found that at the end of ten days a finely powdered article has 

 been broken up to a great extent. It has therefore been thought best 

 to determine the soluble arsenious oxid in Paris green by extracting 1 

 part with 1,000 parts of water (free from carbon dioxid) for ten days 

 and at the end of this time determine the arsenious oxid in an aliquot 

 portion of the filtrate. This method does not distinguish between the 

 three causes of the soluble arsenious oxid, but it is undoubtedly true 

 that all three are bad, although very likelj^ not equally so. Other 

 work done by Avery and Beans distinguishes to a certain extent 

 between free arsenious oxid as such and the other two forms, so that 

 at least we can say whether the soluble arsenious oxid is due to free 

 arsenious oxid on the one hand or badly made or fine Paris greens on 

 the other. 



Since these three distinct types of soluble arsenious oxid have not 

 been previously recognized, and since, when 4 per cent was adopted 

 as the maximum limit of soluble arsenious oxid allowable in Paris 

 green, a much shorter time of extraction with water was used than 

 ten days (resulting in much lower figures for soluble arsenious oxid), 

 it appears to the author that work along this line is needed badly, 

 work that will determine the maximum percentage of soluble arseni- 

 ous oxid that can be present in Paris green without scorching in each 

 of the three forms, or as many of these forms as our present chemical 

 methods will enable us to recognize. 



The Bureau of Chemistry, U. S. Department of Agriculture, in 

 collaboration with the Division of Entomology, is now carrying on 

 preliminarj'^ work of this character, but in order to make the work a 

 success each of the States will have to take it up, it being well known 

 that climatic conditions markedly influence the amount of free arseni- 

 ous oxid that a plant can stand. I therefore lay this matter before 

 you with the hope that entomologists from the various States may be 

 interested enough in the work to collaborate with us during the com- 

 ing spring. The Department will prepare all samples and send them 

 out and the various entomologists will do the spraying, for which full 

 credit will be given in each case. 



The second class of insecticides to which your attention is directed 

 are the London purples. These are prepared by boiling an analine 

 dye residue containing arsenic with lime. Up to a very recent date 



aJour. Amer. Chem. Soc, Vol. XXIII, No. 2. 



