315 



mains. Dr. Thomson analysed it by solution in water, saturation 

 with carbonate of soda, and precipitation by solutions of baryta and 

 silver ; and states its composition from such analysis, to be chromic 

 acid and chlorine, atom to atom. 



The fifth section of this paper is devoted to an account of the salts 

 of chromium. 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 hydrogen. Gallic acid precipi- 

 tates them green. Prussiate of potash only changes their colour to 

 brown, and throws down no precipitate. Ammonia and potash throw 

 down green oxide, which re-dissolves in excess of the latter. 



The muriate of chromium always contains an excess of acid, and 

 is deliquescent. When this is driven off it becomes a chloride, and 

 is insoluble in water and in acids. 



The nitrate also contains an excess of acid, which cannot be neu- 

 tralized by adding more oxide. Dried and slowly heated, it is chiefly 

 converted into chromic acid contaminated with a little green oxide. 



The sulphate also reddens vegetable blues. It consists of the acid 

 and oxide, atom to atom, and 3 atoms of water. 



The precipitate from muriate of chromium by carbonate of soda, 

 is a di-carbonate with 4 atoms of water. When bi-carbonate of potash 

 was the precipitant, a penta- carbonate was obtained, which the 

 author regards as a fact not easily explained. 



The bi-phosphate precipitates from the muriate on adding phos- 

 phate of soda : it is a lively deep green powder ; when dried in a sand 

 heat it contains 5 atoms of water. Neutral phosphate could not be 

 formed. The arseniate is very nearly neutral, but a bin-arseniate 

 also exists. 



Chromic acid dissolves protoxide, and forms a chromate. This is 

 precipitated when muriate of chromium and chromate of potash are 

 mixed, and is of a brown colour, and soluble in water. 



After describing other salts, as the oxalate, tartrate, and potash- 

 tartrate, Dr. Thomson proceeds to give an account of certain chro- 

 mates not before described. These are the per-chromate of iron, the 

 di-chromates of lead and silver, and the double chromates of potash 

 and soda and of potash and magnesia. 



The author concludes this paper with an account of his analyses 

 of the mineral compound known in cabinets as chromate of iron, 

 which, when examined in a state of purity, he found to consist of 

 2 atoms of green oxide of chrome, 1 of peroxide of iron, and 1 of 

 alumina, together with a minute admixture of a white matter, ap- 

 parently a metallic salt, of unknown acid and base, and which, though 

 accompanying specimens from several localities, yet in all was too 

 small in quantity for thorough examination. 



