THE 



PHILOSOPHICAL MAGAZINE 



AND 



ANNALS OF PHILOSOPHY, 



[NEW SERIES.] 



FEBRUARY 1828. 



XIII. On the mutual Decomposition of Sulphate of Zinc and 

 Chromate of Potash. By T. Thomson, M.D. F.R.S. L. # E. 



Professor of Chemistry in the University of Glasgow. 



To the Editors of the Philosophical Magazine and Annals. 

 Gentlemen, 

 TN the last Number of your Journal (for December 1827) I 

 * observed a paper by Mr. Henry Stokes " On some new 

 double chromates." The three compound salts which he de- 

 scribes as double chromates, in fact contain merely a small 

 quantity of chromate of potash, or rather of chromic acid in a 

 state of mechanical mixture. They are the three salts which 

 I have described in my First Principles of Chemistry, vol. ii. 

 under the names of Potash-sulphate of zinc, p. 435, 

 Potash-sulphate of nickel, p. 434, 

 Potash-sulphate of copper, p. 436, 

 as any one may satisfy himself by comparing my descriptions 

 and analyses of these salts with those of Mr. Stokes. 



I shall take this opportunity of rectifying the account which 

 I have given of chromate of zinc in my First Principles of 

 Chemistry, vol. ii. p. 357. The salt which I describe as a 

 chromate of zinc is in reality a dichromate of zinc, for it con- 

 tains exactly twice as much oxide of zinc as I have assigned 

 it. This mistake I discovered soon after the publication of 

 my First Principles. And it induced me to examine carefully, 

 what takes place when solutions of sulphate of zinc and chro- 

 mate of potash are mixed in the atomic proportions. The re- 

 sult is curious, and will be sufficiently understood by the fol- 

 lowing statement. If we mix together solutions containing 



16 atoms chromate of potash = 200 



16 atoms sulphate of zinc = 164 364. 



New Series. Vol. 3. No. 14. Feb. 1828. M There 



