Royal Society. 453 



lar compound of chromic acid and clilorine, discovered some years 

 ago by the author ; but in the investigation to which it gave rise, 

 the author was led to a more careful exanjination of the oxides of 

 chromium than they had before undergone, and to a knowledge of 

 their composition. The communication begins with a description 

 of metallic chromium. That used by the author was reduced by 

 Mr. Cooper. It was white, wuth a shade of yellow, very brittle, 

 and sensibly attracted by the magnet even in fine powder ; its 

 specific gravity 5-093 at least. Nitric acid boiled on it has no ef- 

 fect; and aqua-regia scarcely any, unless the action be very long 

 continued. When heated however with a mixture of potash and 

 nitre, it is converted into chromic acid. The weight of its atom 

 is 4. — The author next describes the green oxide of chromium, 

 which consists of 1 atom metal, 4 and 1 oxygen .1 = 5. And after 

 the description also of two compounds, the one of chromic acid 

 and oxide of chromium, the other of sulphur and the same oxide, 

 he proceeds to describe a combination of 4 atoms peroxide of iron 

 and 1 green oxide of chromium. Phosphuret of chromium he states 

 to contain 1 i atom phosphorus and 1 atom metal. The brown oxide 

 appears to be either a mixture, or a compound far from intimate, 

 of I atom chromic acid and 6 atoms of oxide of chromium. 



The next section of this paper is devoted to an account of the 

 chloro-chromic acid, a remarkable compound produced by making 

 sulphuric acid acton a mixture of 190 parts of bichromate of potash, 

 and 225 parts of common salt. From this mixture, on applying 

 heat, it separates in red fumes, and distils over in a liquid of a rich 

 deep crimson colour, of a sweet, astringent, acid taste, and having 

 a strong smell of chlorine. Specific gravity nearly twice that of 

 water, with which it does not mix, but which decomposes it, evol- 

 ving chlorine and producing heat. This liquid, when dropped into 

 oil of turpentine or alcohol, or poured on sulphur, sets them on 

 fire; but (which is remarkable) it not only does ho^ fire phosphorus, 

 but even extinguishes it when already inflamed. On other com- 

 bustibles and metallic bodies it acts with great energy, but without 

 producing ignition : in ammoniacal gas, however, it burns readily. 

 When heated, /7er se, the chlorine escapes, and a substance resem- 

 bling green oxide remains. Dr. Thomson analysed it by solution 

 in water, saturation with carbonate of soda, and precipitation by 

 solutions of baryta and silver; stating its composition to be an atom 

 of chlorine and an atom of chromic acid. 



An account of the salts of chromium occupies the succeeding 

 portion of the communication. They are formed by the union of 

 the green oxide with acids, and are all uncrystallizable and of very 

 intense colours. They are not precipitated by sulphuretted hy- 

 drogen ; gallic acid precipitates them green ; prussiate of potash 

 only changes their colour to brown, and throws down no precipi- 

 tate ; ammonia and potash throw down green oxide, which redis- 

 solves in excess of the latter. — The following salts are next de- 

 scribed in detail : the muriate, nitrate, sulphate, dicarbonate, bi- 

 phosphatc, chromate, oxalate, tartrate, and pota.sh-tartrate of thro- 

 iiiium. — Dr. T. then gives an account of certain chrnmatos not be- 

 fore 



