313 



partly from errors in the solar tables, and partly from instrumental 

 derangement. 



On some of the Compounds of Chromium. By Thomas Thomson, 

 M.D. F.R.S. L. and E. Professor of Chemistry, Glasgow. Read 

 March 29, 1827. [Phil. Trans. 1827, p. 159.] 



The principal object of this paper is to give an account of a sin- 

 gular 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 chro- 

 mium 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 pre- 

 cipitation 16'23 of chromate of lead, giving for the weight 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 al- 

 cohol, sulphurous acid, or sulphuretted hydrogen, on chromate of 

 potash. When thus obtained it is in the state of a hydrate, contain- 

 ing -} fth.3 of its weight of water, and easily soluble in acids. A mo- 

 derate heat, however, expels the water, and leaves the oxide inso- 

 luble 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 cry- 

 stallizable 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 chro- 

 mate of potash by sulphuretted hydrogen. Hydrosulphuret of chro- 

 mium, 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, -f- 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 



