On some of the compounds of chromium

The principal object of this paper is to give an account of a singular compound of chromic acid and chlorine, discovered some years ago by the author; but in the investigations to which it gave rise, the author was led to a more careful examination of the oxides of chromium than they had before undergone, and to a knowledge of their composition. An account of these researches he therefore proposes to give in this communication. He begins by describing metallic chromium. That used by him was reduced by Mr. Cooper: it was white, with a shade of yellow, very brittle, not sensibly attracted by the magnet even in fine powder. Its specific gravity was 5.093. Nitric acid boiled on it has no effect, 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; 3.14 grains of the metal, thus treated, yielded by solution and precipitation 16.23 of chromate of lead, giving for the w'eight of an atom of chromium 3'966, or in round numbers 4.000. The author’s stock of metallic chromium was so small as to prevent the repetition of the experiment. The author next describes the green oxide of chromium. This is easily produced by the action of de-oxygenizing agents, such as alcohol, sulphurous acid, or sulphuretted hydrogen, on chromate of potash. When thus obtained it is in the state of a hydrate, containing 1/1 3/5 ths of its weight of water, and easily soluble in acids. A moderate heat, however, expels the water, and leaves the oxide insoluble in any acid. When further heated nearly to redness, it glows, or becomes of itself suddenly intensely red hot. Its atomic weight cannot be determined from its salts, as it forms none,—at least crystallizable and definite enough for the purpose ; but as we know that of chromic acid to be 6.5, if we can determine the number of atoms of oxygen to be abstracted to convert it into green oxide, that of the latter will be known. To this end the author deoxidized the chromate of potash by sulphuretted hydrogen. Hydrosulphuret of chromium, composed of its ingredients, atom to atom, fell in the state of a green powder soluble in acids. The liquor, after driving off the redundant gas by heat, was found to be a solution of hyposulphite of potash. To avail himself of this fact, however, it became necessary to investigate the composition of the hyposulphurous acid. This he effected as follows:—he first analysed a crystallized hydrosulphuret of soda, formed on a large scale in certain soda-leys, which he found to consist of 1 atom bisulphuretted hydrogen, + 1 atom soda, + 6 atoms water. Through a solution of this salt he passed sulphurous acid, which converted it into hyposulphite, and threw down just half the sulphur contained in the salt. The hyposulphite of soda thus obtained gave by analysis a per-centage of hyposulphurous acid, agreeing with 5 as its atomic number, on which supposition it must be regarded as consisting of 2 atoms sulphur, + 1 oxygen; and this composition he states himself to have verified by direct analysis of several hyposulphites. From this it is easy to derive the composition of the green oxide of chromium, the weight of which thus comes out equal to 5.