Mr. Stokes on some fiew double Chromates. 427 



they exist in chemical combination ? I am inclined to think 

 that these crystals are not fluo-iodic acid, and I am convinced 

 that they contain no ammonia; for on pouring sulphuric acid 

 into the solution (A) boiling in a platina crucible, I observed 

 a yellowish gaseous substance arise, which condensed in the 

 form of a yellow crust on the sides of a glass receiver : having 

 dissolved this crust in water and evaporated the solution, I 

 obtained yellow crystals perfectly similar to those already de- 

 scribed. 



I thus send you the imperfect results of experiments still 

 more imperfect; but being on a journey, I am unable to pursue 

 these researches as far as I could wish. But if you should 

 esteem my attempts not altogether useless, you will greatly 

 oblige me bv giving thenj a place in your Journal ; for it seems 

 to me no abuse of the attention of chemists to direct it to new 

 objects of research. 



LXXII. • On some nevo double Chromates. By Mr. Henry 



Stokes*. 

 TN Dr. Thomson's " First Principles of Chemistry," when 

 -■" speaking of chromate of zinc, iie says : " this salt was ob- 

 tained by mixing solutions of sulphate of zinc and chromate 

 of potash in the atomic proportions. The chromate of zinc 

 precipitated in the state of a yellow powder. The supernatant 

 li(|uid being still yellow, was concentrated. It yielded two dis- 

 tinct sets of crystals intimately mixed with each other; namely, 

 bichromate of potash and sulpliate of zinc, tinged yellow by 

 chromate of potash." Vol. ii. p. 357. 



On preparing by this process some chromate of zinc, a good 

 deal of this yellow salt formed, along with bichromate of pot- 

 ash : but on examining the form of the salt, I immediately 

 saw that it was not that of sulphate of zinc, being a flat rhombic 

 prism, with the acute angles truncated. It being hence evi- 

 dent that it was a distinct salt, I set about examining it. 



In the first place it was obvious from its yellow colour that 

 it contained chromic acid. When an acid solution of nitrate 

 of barytes was added, a white precipitate fell down, showing 

 the presence of sulphuric acid; carbonate of soda occasioned 

 a white flocculent precipitate of carbonate of zinc : potash was 

 also suspected to be present. 50 grains of the crystals were 

 dissolved in distilled water, and a solution of nitrate of silver 

 added while any precipitate fell down : this precipitate of 



• Communicated by the Autiior. 



3 I 2 chromate 



