Mr. Croft on a new Oxalate of Chromium and Potash. 197 



imagination can conceive : the red crystals do not appear to 

 be affected by polarized light, so far as the display of colour 

 is concerned. 



The magnifying powers used in these investigations were, 

 for the experiments on the sublimed crystals, 200 times linear 

 measurement or diameters; in the precipitated compound, 

 620 diameters. 



XXXIV. On a new Oxalate of Chromium and Potash. By 

 Henry Croft, Esq.* 



IT is well known that in 1830 Wilton Turner accidentally 

 discovered a salt composed of oxalate of the oxide of chro- 

 mium and oxalate of potash. Its curious optical properties 

 have been examined by Brewsterf. Gregory also discovered 

 the same salt independently, and proposed a much better me- 

 thod for obtaining it than that used by Turner, which con- 

 sisted in adding oxalic acid to a solution of bichromate of po- 

 tash until effervescence ceased : the solution became deep 

 green or black, and on evaporation yielded beautiful crystals 

 of the black salt. Gregory supposed it to consist of 3 equi- 

 valents of oxalic acid, 2 of potash, 1 of oxide of chromium, 

 and 6 of water. Its true composition, 3 (KO, C 2 O a ) 4- Cr 2 O a , 

 3 C 2 3 + 6 HO has been shown by Graham and Mitscher- 

 lich, who have also prepared a number of salts similarly con- 

 stituted. 



On attempting to prepare the black salt by Turner's method 

 I could never completely succeed, but obtained in its stead, 

 when a very concentrated hot solution of the bichromate was 

 employed, a red granular precipitate, which proved to be a 

 new salt, and forms the subject of the present notice. 



Perhaps the best method of preparing it is that above de- 

 scribed, viz. to employ as concentrated a solution of the bichro- 

 mate as possible, in which case the salt crystallizes out on 

 cooling. The precipitated salt must be redissolved in a small 

 quantity of water and allowed to crystallize. It is however 

 one of the most difficult salts to crystallize that is known : in 

 nine cases out of ten it separates in the form of a somewhat 

 granular bluish gray powder, and it appears to be only under 

 particular circumstances that it will crystallize well, which, 

 however, I was not able to discover. It does not seem to 



* Communicated by the Chemical Society, having been read February 15, 

 1842. 



[t See Phil. Mag. Third Series, vol. vii. p. 436. Some of the optical and 

 crystallographical properties of this salt have also been described by Mr. 

 Talbot, in Phil. Mag. Third Series, vol. x. p. 218, and vol. xiv. p. 21.— 

 Edit.] 



