OF ARTS AND SCIENCES. 195 



of potash has been added. By accurate quantitative experiments we 

 have proved that five equivalents of chromate of potash must be added 

 to one equivalent of chi-ome alum to effect precipitation. If a less 

 quantity be added, any precipitate which may form will at once redis- 

 solve when the mixture is made complete by agitation.* The reaction 

 between the two salts may therefore be expressed by the formula, 



5 (KO CrOs) + KO SO3, Cr^O^ 3 SO3 = 4 (KO SO3) + 2 (KO 2 CrOs) 

 + Cr303Cr03, 



and if to one equivalent of chrome alum more than five equivalents of 

 chromate of potash be added, the excess above five remains inactive. 



X (KO Cr03) + KO SO3, Cr^Og 3 SO3 = 4 (KO SO3) + 2 (KO 2 CrOg) 

 + Cr^Oa CrOa + (x — 5) (KO Cr03) . 



If any other normal salt of chromic oxide be used, a similar formula 

 will express the reaction ; thus : — 



Cr^Cls + 5 (KO CrOa) = 3 (K CI) + 2 (KO 2 CrOs) + ^^03 CrOj. 



CrA 3 SO3 + 5 (KO CrOs) = 3 (KO SO3) + 2 (KO 2 CrOs) + 

 Cr^Oa Cr03. 



With the exact proportions of chrome alum and chromate of potash 

 which are by the formula necessary for the precipitation of chromate of 

 chromium we prepared a quantity of the precipitate for a second series 

 of analyses in corroboration of analyses a, h, and c. The precipitate 

 thrown upon a filter was very slightly rinsed with cold water, a por- 

 tion of it was taken off, pressed between folds of filter-paper under 

 heavy weights, and air-dried for analysis (analysis d) ; the rest of the 

 precipitate was washed for six hours till no trace of the filtrate could 

 possibly be supposed to be retained by the precipitate, when another 

 portion was taken out for analysis, pressed, and aii'-di-ied (analysis e) ; 

 lastly five htres of water were passed through the i*emaining precipi- 

 tate, which was then dried at 100° and submitted to analysis (analy- 



* In this connection we would call attention to the inaccuracy of the statement 

 made by Berzelius in his Traite de Chimie, (Paris, 1846, Vol. II. 307), to the 

 effect that a compound whose formula is Cr203 3 CrOa is precipitated, when a few 

 drops of the solution of monochromate of potash ai-e added to the solution of a 

 neutral chrome salt. No permanent precipitate whatever is produced under these 

 circumstances, as any one may satisfy himself by repeating the simple experiment 

 described above. 



