PARIS GREENS. 15 



by tlu 1 following method in the hope that at some future time the rela- 

 tion between the copper oxid dissolved and the Paris Green broken up 

 might be determined: Another 200 cc of the above solution is treated 

 with 5 cc of hydrochloric acid, heated to 70 C., and a current of 

 hydrogen sulphid passed through until all arsenic and copper are pre- 

 cipitated. It is then filtered and washed quickly with hot water. The 

 filter and its contents are burned in a porcelain crucible with powdered 

 sulphur and weighed. In this way a mixture of CuO and Cu 2 S is 

 obtained. To calculate the amount of CuO dissolved, consider the 

 residue as entirely made up of CuO. This is correct, since the per- 

 centage of copper in CuO and Cu 2 S is the same. 



The second or sodium-acetate-extraction method for determining the 

 free arsenious oxid in Paris Green is made as follows: Digest over 

 the flame 1 gram of Paris Green for about 5 minutes with 25 cc. of a 

 solution of sodium acetate containing 12.5 grams of the crystallized 

 salt. The solution is then cooled, made up to 100 cc. and 50 cc. filtered 

 off and titrated with standard iodin in the usual way. a 



It will not be out of place at this point to discuss briefly these two 

 methods of analysis. It appears from the work that has been done on 

 the subject that the sodium-acetate method gives more closely the true 

 percentage of free arsenious oxid in the sample of Paris Green, while 

 the water-extraction method gives the percentage of free arsenious 

 oxid in the green plus some arsenious oxid obtained by the decompos- 

 ing action of the water on the Paris Green. In some very coarse 

 samples of Paris Green, which are supposed not to be made as well as 

 the finer samples, the author found recently that the sodium -acetate- 

 extraction method gave very low figures for free arsenious oxid (show- 

 ing that only a small amount of free arsenious oxid as such was present), 

 while the water-extraction method gave very high figures (seeming to 

 show that although only a small amount of arsenious oxid was present 

 in the free state, there was present a certain portion of the green, 

 which was in a very loose combination, and consequently was easily 

 broken up). That portion of the green which was in such a loose com- 

 bination would, in all likelihood, when applied to the plant soon break 

 up and scorch the foliage. It therefore appears that while the sodium- 

 acetate-extraction method, as before mentioned, gives more closely the 

 actual percentage of free arsenious oxid present in the green, the water- 

 extraction method gives some idea of its stability, and consequently 

 will express more closely the value of the compound in actual orchard 

 practice. It has recently been called to the author's attention by Mr. 

 C. L. Marlatt, Assistant Chief of the Division of Entomology, that sam- 

 ples of Paris Green which do not scorch may be made to do so by 

 grinding them up to a very fine powder and then applying them to 



See paper of Avery and Beans, Jour, of the Amer. Chem. Society, Vol. XXIII, 

 No. 2. 



