9(5 PROCEEDINGS OF THE AMERICAN ACADEMY 



7.88% — 17.82% of acetic anhydride, a variance of nearly 10%. 

 As the salt is crystalline, and is formed out of different propor- 

 tions of the materials used, with the same composition, as it was in 

 several cases where no great change was made, it appears as a definite 

 chemical compound, and the variation is, I think, best explained by 

 considering the existence of several definite salts. Also in three cases 

 I have found water chemically combined, which shows still more vari- 

 ance, but these salts also appear definite, and agree with vei*y simple 

 formulas. 



Pure Schweinfurt green may therefore be composed either entirely 

 of the salt having the formula (CuOCJIuO.) 3(CuOAs03), or en- 

 tirely of that having the formula 2(CuOC^HP3) 3(CuOAs203), or 

 may have a composition between the two, from a mixture of these 

 salts, and it may contain combined water having this formula, 

 2(CuOC,H,03), 5(CuOAs20,3)4-2H,0, or may have the formula 

 2(CuOC,H,03) 3(CuOAs203) 5H,0. And all tiiese results may be 

 varied again still more by a mechanical mixture of arsenic trioxide, 

 from the difficulty of removing the same by washing. That the arsenic 

 is present as trioxide, it contains no arsenic pentoxide so far as I have 

 been able to find in this investigation. The salts of the formulae 

 2(CuOC,H,03) 3(CuAs,03) and 2(CuOC,HA) 3(CuOAs,03)+5H20 

 are more brilliant than the other two with less basic copper acetate 

 in them, the difference being quite marked. 



Therefore this beautiful pigment is not so definite as has been pre- 

 viously supposed, but may vary some ten per cent in its amount of 

 acetic acid, twelve per cent in its arsenic trioxide, and five per cent 

 in its amount of copper oxide ; while some samples contain chemi- 

 cally combined water, in one specimen going up to seven per cent. 

 And all of these would be taken by an observer for Schweinfurt 

 green, and can only be distinguished by a slight difference in shade of 

 color and chemical composition, which latter shows there exists three 

 definite salts. 



In the analyses of this paper, the following methods were employed: 



A weighed portion was dissolved in an excess of nitric acid, the 

 solution boiled some time in order to oxidize the arsenic trioxide, 

 the solution was then made alkaline with caustic potasli, which pre- 

 cipitated the copper as oxide, which if the acids are all oxidized is 

 black; if not, orange-red, in which case the separation is not as per- 

 fect, and it cannot be manipulated with as much ease, as when the 

 oxidation is perfect. The fluid with the precipitate is boiled and 



