136 On the Combinations of Chromium. 



Art. XX. — On the Combinations of Chromium; by Augus- 

 tus A. Hayes. 



In the published accounts of the combinations of this 

 metal, much discordance exists, and the processes recom- 

 mended for procuring the oxides, are not only ineligible, but 

 fallacious. The memoir of Vauquelin, contains nearly all 

 that is known on this interesting subject, notwithstanding the 

 time which has elapsed, since the discovery of the metal. 

 The following additional observations, connected with its 

 chemical history, are submitted by the writer. 



Protoxide of Chromium. — This oxide is of a dark green 

 color, when in fragments it possesses considerable lustre, it 

 is soluble in the strong acids, and forms with them green so- 

 lutions ; it is less fusible than platina. When mixed with 

 three parts of nitre and exposed to heat, it is acidified and 

 forms chromic acid, which, uniting to a part of the potash 

 present, forms bichromate of potash. When heated nearly to 

 redness it becomes suddenly ignited, and presents the appear- 

 ance of combustion; an effect first observed by M. Berzelius, 

 and by him referred to internal change of form, consequent 

 to rapid contraction. This interesting character is exhibi- 

 ted, when we place a few grains of the dry oxide in a pla- 

 tina spoon, gently press it, so as to distribute it equally over 

 the surface and cause it to slightly cohere, and subsequently 

 heat it by the blow pipe flame ; the oxide is partly dispersed 

 by the sudden action. If considerable quantities of the ox- 

 ide be heated in a capsule, the ignition commences at. the 

 surface in contact with the disk, and sufficient heat is devel- 

 oped, to continue the action throughout the mass. The 

 color is usually changed to a light green, and long ignition ren- 

 ders it nearly grass green. It is obtained by heating the 

 hydrate to 400° F. A light green, insoluble oxide is obtain- 

 ed in prismatic fragments, by heating tartrate of chromium 

 to redness. 



Protohydrate of Chromium. — This compound is of a 

 greenish black color, brittle, and the fracture exhibits a vit- 

 reous lustre. It is soluble in strong and diluted acids. When 

 recently prepared, it is soluble in a solution of pure potash, 

 which, when saturated, is of a dark green color; the oxide 

 precipitates as the solution of potash, absorbs carbonic acid 

 from the atmosphere. Ammonia in solution, does not dis- 



