184 EXPERIMENT STATION. [Jan. 



allowed to stand two hours, then well stirred, after half an hour 

 stirred again, and finally at the end of a quarter of an hour the 

 liquid is drawn off and filtered. The resulting Paris green is 

 dried on racks for four or five days at 185° F., or in a steam 

 vacuum oven about 2G0° F. The yield is 985 pounds. 



An electrolytic process for making Paris green from metallic 

 cojDper, arsenous oxide and acetic acid was patented by Richard 

 Franchot in 1902. No information relative to the character of 

 the product is available. 



Paris green is a copper aceto-arsenite for which Eugene 

 Ehrmann's formula ^ is generally accepted. 



Cu(C2H302)2 • 3 Cu(As02)2. 



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

 acetate to 3 of copper metarsenite, equivalent to 17.91 per cent, 

 of the former to 82.09 per cent, of the latter. The structure of 

 Paris green and its homologues was carefully studied by Avery, 

 and while his results ^ most frequently approached a ratio of 

 1:3, there was invariably a deficiency in arsenic. As the prod- 

 uct is not recrystallizable he recognized that purity must be 

 assured largely by a microscopical examination, which proved 

 a questionable guide for so unstable a compound. 



Although some chemists claim that the formula is only em' 

 pirical it certainly expresses the proportion of cupric oxide to 

 combined arsenic trioxide as found in well-formed greens. Four 

 hundred and ninety-nine samples ^ collected in the open market 

 by the Pennsylvania department of agriculture contained on the 

 average : — 



Per Cent. 



Cupric oxide, 29.41 



Total arsenic trioxide, 56.56 



Water soluble arsenic, 1.41 



The relation of cupric oxide to " insoluble " arsenous oxide is 



1 :1.875, theory 1 :1.865. Similar results are reported by others. 



The comparatively high specific gravity of Paris green, as 



' Bui. Soc: ind.. Mulhausen 7, pp. 68-80 (1834). 



2 Jour. Amer. Chem. Soc, 28, p. 1155 (1906). 



3 J. W. Kellogg, Bui. No. 192, p. 37 (1910). 



