On some Chromates. 283 



it has also been examined by Mitscherlich ; vide Lehrbuch, 

 vol. ii. part 2.] 



Hydrated oxide of chromium dried at 100° contains six 

 atoms of water. Schrbtter also states that the green modifi- 

 cation of chrome alum when in solution passes gradually into 

 the blue one. [This- statement I can fully confirm from my 

 own old observations. This change of the green into the blue 

 oxide accounts for Warington's preparation of the double 

 oxalates of chromium and potassa by means of green oxide of 

 chromium*, although from the mode of preparation it is evi- 

 dent that the blue oxide is the base in the black and red ox- 

 alates. — H. C] 



On some Chromates. 



Kopp has examined several of these salts, principally with 

 a view to determining their specific gravities and atomic 

 volumes. The chromates of zinc and copper may be obtained 

 by dissolving the oxides or carbonates in dilute chromic acid 

 (prepared by Fritzsche's method), or by digesting chromate 

 of baryta with the sulphates [several salts of the magnesian 

 class were prepared some years since in this manner by Mr. 

 Play fair, but no account of them has been published]. The 

 salts of copper and zinc have the same form and composition 

 as the sulphates of those oxides ; the soda salt is similar to the 

 sulphate, it deliquesces. By evaporating its solution at 30° C. 

 anhydrous chromate may be obtained. The chromates of am- 

 monia and magnesia are precisely similar to the corresponding 

 sulphates. — (Annalen der Chemie, Sfc, vol. xlii. p. 97.) 



Benschhas published a notice on some basic chromates ob- 

 tained by pouring a solution of chromate of potassa into boil- 

 ing neutral metallic solutions. These precipitates must be 

 washed with hot water, or else they retain some potassa ; by 

 boiling they appear to be decomposed. None of them have 

 been properly examined as yet ; the manganese salt is black, 



its formula is Mn 2 Cr + 2 aq. When heated red-hot the 

 water and some oxygen are driven off. — (PoggendorfFs An- 

 nalen, vol. 1. p. 97.) 



[The same salt appears to have been formed by Mr. Wa- 

 rington (Reports of the Chem. Soc, part 3), who has obtained 

 the same formula. Salts of protoxide of manganese are white 

 or pinkish ; the salts of chromic acid are seldom very dark- 

 coloured, and it appears rather anomalous that this basic salt 

 should be black. It might be Cr 2 + Mn 2 3 + 2aq; when 

 treated with hydrochloric acid the sesquioxide of manganese 

 would cause evolution of chlorine, and a brown chloride of 

 chromium might be produced, which by the addition of alco- 

 * See p. 201 of the present volume.— Edit. 



