and sulphurous acid is driven off, at least if heat be applied to the liquid. A careful analysis of this salt gave its constituents as follows: It is obvious, that the hyposulphurous acid in this salt is a compound of 2 atoms sulphur and one atom oxygen, and that its atomic weight is 5. It is equally obvious, that the hydrosulphuret employed in its fabrication contains not sulphuretted hydrogen, but bisulphuretted hydrogen. For, let us suppose a mixture to be made of 14.875 of the hydrosulphuret of soda and 4 sulphurous acid, 2 of sulphur will precipitate, and a neutral hyposulphite of soda will be formed. 14.875 hydrosulphuret contain 2 atoms sulphur and only 1 atom hydrogen. There remain in solution (as half the sulphur falls) 1 atom sulphur + 1 atom hydrogen. The 4 sulphurous acid are composed of 1 atom sulphur + 2 atoms oxygen. One of these atoms of oxygen in the sulphurous acid will combine with the atom of hydrogen in the hydrosulphuret, and form water; so that there remain for the constituents of the hyposulphurous acid, 2 atoms sulphur 1 atom oxygen 5 There cannot have been more than 1 atom of hydrogen in the hydrosulphuret. For had there been two atoms, consti stituting a bihydrosulphuret of soda, the whole oxygen of the sulphurous acid would have been converted into water, and the hyposulphurous acid would have contained no oxygen at all. Such is the composition of the hyposulphurous acid of HERSCHEL. I verified this constitution by the direct analysis of several hyposulphites; the most easily analyzed of which were the hyposulphites of barytes and of lead.* The reader being now aware of the composition of hyposulphurous acid, will be prepared for the theoretical explanation of the decomposition of chromic acid by sulphuretted hydrogen gas. Let us suppose a solution of 125 grains of chromate of potash, containing 65 grains (equivalent to 10 atoms) of chromic acid. To reduce this to green oxide, 5 atoms of sulphuretted hydrogen gas are requisite, containing 5 atoms sulphur, 5 atoms hydrogen. The sulphur is converted into sulphurous acid and must combine with 10 atoms oxygen, The hydrogen unites to forming water. 5 atoms oxygen, 15 Thus the 10 atoms chromic acid, in order to become green oxide, must part with 15 atoms of oxygen; or (which is the same thing) 1 atom of chromic acid is converted into green oxide when it is deprived of 1.5 atom oxygen. But chromic atom There is another acid which exists, composed of 1 atom sulphur + oxygen. Its atomic weight is 3. It may be distinguished by the name of subsulphurous acid. acid weighs 65; and 65-1'5 = 5 atomic weight of green oxide. The 5 atoms of sulphurous acid thus formed unite with 5 atoms of sulphuretted hydrogen; and these two bodies are mutually decomposed into 5 atoms hyposulphurous acid and 5 atoms water. The 5 atoms of hyposulphurous acid uniting to the 10 atoms of potash constitute dihypo-sulphite of potash, composed of 1 atom hyposulphurous acid 5 2 atoms potash 12 17 I conceive that this beautiful example of rather a complicated series of decompositions and new combinations, leaves no doubt that the atomic weight of green oxide of chromium is 5. When we take bichromate of potash instead of neutral chromate, and treat it with sulphuretted hydrogen gas, the precipitated hydrosulphuret has a buff colour instead of a green. This may perhaps be ascribed to a portion of undecomposed chromic acid falling in combination with some part of the precipitate. For we shall see afterwards a similar coloured precipitate composed of chromic acid and green oxide. This buff coloured precipitate is tasteless, and insoluble in water. It is either a hydrosulphuret of chromium, or at least contains a hydrosulphuret; for when it is heated sulphur sublimes, and sulphurous acid is given off. At a certain temperature it catches fire, and burns for some time with a yellow low flame. When digested in muriatic acid some sulphur precipitates. When digested in nitro-muriatic acid, a portion of the sulphur is converted into sulphuric acid. From 7:71 grains of the buff coloured precipitate I obtained It was easy to drive off water, but not to determine its quantity. The 5'02 of green oxide contained th of a grain of chromic acid. If we suppose the sulphur to be combined with hydrogen, and what is wanting to make up the weight to have been water, the constituents of the buff-coloured powder will be The whole chromic acid, owing to an accident, was not collected; but from some experiments to be related afterwards, I am disposed to view the buff-coloured powder as a compound of 5 atoms hexa-chromate of chromium* 12 atoms water 36.5 7.125 13'5 57.125 Green oxide of chromium, when prepared by the usual processes, is not always free from chromic acid; but in consequence of the property which sulphuretted hydrogen has of reducing chromic acid to green oxide, we have it always in our power to free it entirely from all such admixture. For this purpose it may be dissolved in muriatic acid, and a * By hexa-chromate of chromium, I mean a compound of i atom chromic acid, and 6 atoms green oxide of chromium. current of sulphuretted hydrogen may be passed through the solution, previously rendered as neutral as possible by evaporating it to dryness, and re-dissolving the residual matter in water. If the green oxide was pure, no precipitate appears; but if chromic acid was present, a quantity of the above described buff-coloured precipitate will be thrown down. When sulphate of chromium in the state of a dry powder is put into a glass tube and heated to redness over a spirit lamp, while a current of hydrogen gas is made to pass over it, water is at first given off, and afterwards sulphuretted hydrogen. When all action is at an end, a black, tasteless, insoluble matter remains in the tube, composed (judging from the loss of weight) of It is therefore a disulphuretted oxide of chromium; or, perhaps, a dihydrosulphuret of chromium. 2. I shall now relate the experiments by which chromic acid was converted into green oxide of chromium, by means of protosulphate of iron. When a solution of protosulphate of iron is mixed with one of chromate of potash, a buff-coloured precipitate falls, which is a combination of peroxide of iron and green oxide of chromium. In this case, the protoxide of iron is peroxydized at the expence of the oxygen in the chromic acid, which becomes green oxide. Thus we have it in our power to convert chromic acid at once to green oxide, by mixing it |