1826.] to Dr. lire's Review. II 



ftounced impossible, ex cathedra, is as follows : Chroraate of 

 potash is dissolved in water, and tartaric acid is added to the 

 hot solution till it acquires a green colour. Ammonia added to 

 this solution throws down the protoxide of chromium in the 

 state of a hydrate. It seems the worthy Doctor had been trying 

 his hand at this experiment ; but had been unsuccessful : and 

 such is his high opinion of his own prowess that he deems every 

 thing impossible which he cannot himself perform. But this is, 

 perhaps, rather narrowing the scale of possibility too much ; for 

 I am humbly of opinion that many things are not only possible, 

 but easy, which the Doctor cannot accomplish. Thus I know 

 that my friend Mr. Luke Howard can manufacture most beauti- 

 ful crystals of tartaric acid, and that my friend Mr. Ramsay, of 

 Glasgow, can manufacture excellent crystals of bichromate of 

 potash ; yet if we were to apply the Doctor's test of possibility, 

 we should conclude that neither the acid nor the salt could be 

 manufactured at all. I shall be good-natured enough to explaia 

 the cause of the Doctor's failure, and thus render the prepara- 

 tion of protoxide of chromium by means of tartaric acid and 

 ammonia, a possible process, even when conducted by the 

 worthy Professor of the Andersonian Institution, Glasgow. 



Tartaric acid has the property of combining at once with 

 potash and most other salifiable bases. If you digest bitartrate 

 of potash and protoxide of chromium mixed in the atomic pro- 

 portions, and with the requisite quantity of water, a complete 

 solution will be obtained, having a deep bluish green colour, and 

 almost opaque. When this solution is evaporated to dryness, 

 a greenish black powder remains, which is a compound of two 

 atoms tartaric acid, one atom potash, one atom protoxide of 

 chromium, with a certain quantity of water of crystallization. 

 This salt is readily soluble in water, and like the other compound 

 tartrates is not precipitated by ammonia, nor by the fixed alka- 

 lies or their carbonates. This property of the compound tar- 

 trates has been long known to chemists ; but it is particularly 

 insisted on in a note to a very valuable paper of M. H. Rose, 

 printed in the Memoirs of the Swedish Academy for 1820. 



Dr. Ure had dissolved chromate of potash in water, and had 

 added not merely the quantity of tartaric acid necessary to 

 reduce the chromic acid to the state of protoxide; but enough 

 to saturate the potash and protoxide of chromium in the solu- 

 tion. He had thus formed the compound salt Avhich I call 

 potash-lartrate of chromium. Now from the solution of this 

 salt when it has been once formed, the protoxide of chromium 

 cannot be precipitated by ammonia ; but even when this blunder 

 has been committed, the remedy is easy. Evaporate the liquid 

 to dryness, ignite the dry mass to destroy the tartaric acid, 

 digest the black residue in water to dissolve out the potash. A 

 black powder will remain which is a mixture of charcoal and 



