490 Mr. Warington on the Action of Chromic Acid upon 



pense of the chromic acid of the bichromate of potass, and 

 that the chromic acid must therefore be divided into two parts, 

 the first of which, yielding oxygen to the metallic silver, forms 

 the oxide of that metal, which is directly seized by the se- 

 cond, or undecomposed part of the chromic acid, giving rise 

 to the crimson crystalline salt or bichromate of silver; while 

 at the same time the deoxidized chromic acid or protoxide of 

 chromium of the first part, enters into combination with the 

 sulphuric acid and the potass of the bichromate of potass, to 

 form the chrome alum; the sulphuric acid taking no part in the 

 decomposition, but acting merely as a necessary agent to in- 

 duce the stronger affinities, and to unite with the protoxide of 

 chromium and the potass as soon as liberated from their re- 

 spective combinations. 



For the perfect success of this experiment care should be 

 taken that the plate or capsule of silver employed is rendered 

 perfectly clean, and free from any coating of sulphuret or chlo- 

 ride of that metal. I have usually, after rubbing the surface 

 with a little fine emery powder, washed it with solution of 

 ammonia before bringing it into contact with the mixed solu- 

 tion, and have always found the action to commence imme- 

 diately. The proportions that appear to afford the most com- 

 plete results are one proportion of the bichromate of potass, 

 or about 150 grains, dissolved in water, and three or four pro- 

 portions of liquid sulphuric acid, or from 150 to 200 grains 

 by weight, equal to from 76 to 102 grains by measure. 



The bichromate of silver crystallizes in rhomboidal plates, 

 having frequently two of the angles opposed to each other 

 truncated; it has an acid reaction, is slightly soluble in water, 

 yielding a rich amber yellow solution, which deposits very 

 dark brown crystals by spontaneous evaporation ; these how- 

 ever are crimson by transmitted light, and when rubbed down 

 in a mortar give a crimson powder. 



The bichromate of silver is also formed whenever an acid 

 salt of silver is precipitated by the bichromate of potass, and 

 even at times by the neutral chromate, as is the case with the 

 sulphate of silver ; and although the fact of a crystalline pre- 

 cipitate of chromate of silver being sometimes obtained is 

 mentioned in many of the best works on chemistry, yet the 

 composition, or a difference in composition from the brown- 

 red precipitate of chromate of silver as it usually occurs, is 

 never hinted at. In making this statement, however, I should 

 except the Traite des Essais par la voic scche of M. Berthier, 

 and also a short notice by Mr. Teschemacher in the Phil. Mag. 

 and Annals, N.S. vol. i. p. 34-5. 



20 grains of the bichromate of silver obtained by the pro- 



