On the Combinations of Chromium. 1 41 



blue. Like the tartrate, it cracks into prismatic portions ; 

 they are however generally much smaller; it is readily de- 

 composed by heat, and affords the protoxide. 



Other salts have been formed, but they are either nearly 

 insoluble, or uncrystalizable. In its combinations with 

 acids, protoxide of chromium, presents some of the charac- 

 ters of alumina ; but it does not like that earth, form triple 

 salts, — at least so far as I have investigated its compounds. 



II. On the combinations of chromic acids, with bases. — These 

 compounds are so well known, that it is not necessary for 

 me to enter into details ; — I shall therefore add only a few 

 remarks on their colors, and the processes of obtaining them. 



Chr ornate of Potash. — Its crystals are minute prisms of 

 five and six sides ; its color is lemon yellow ; it is very solu- 

 ble in water. It is obtained by neutralizing a solution of the 

 bichromate ; by subsequent evaporation and rest, the crystals 

 are deposited. 



Bichromate of Potash. — When crystalized without the 

 presence of other salts, it presents rhombic tables, truncated 

 on their obtuse, lateral edges ; its color is pure orange-red ; 

 — crystalized in an acid solution, its color is red brown. Hy- 

 drosulphric acid gas, decomposes this salt, and gives a pre- 

 cipitate of oxide of chromium, mixed with sulphur. 



Chromate of Ammonia. — The crystals are minute prisms, 

 which are aggregated so as to present thin plates, resembling 

 the form of the index used in writing ; its color is yellow, and 

 its lustre is metallic. It is formed by neutralizing chromic 

 acids, by ammonia. It is a beautiful salt. 



Bichromate of Ammonia. — The crystals are rhombic prisms ; 

 color, red-brown ; less soluble than the chromate. When 

 heated on platina foil, at a temperature below redness they 

 are decomposed, with the evolution of light and a slight de- 

 tonation ; oxide of chromium remains. 



Chromate of Iron. — When a solution of proto sulphate of 

 iron, is added to a solution of chromate of potash, the chromic 

 acid of the chromate is decomposed, its oxygen unites to the 

 protoxide of iron forming the peroxide ; a part of which can- 



