1. Fifty grains of residue dried at the temperature of between 170 and 180, (as described in § VI.) and therefore equal to ten ounces of the water, were boiled in successive solutions of potash, so as to saturate all the acid contained in that residue, and to dissolve the alumine. The remaining solid residue (which had passed first to a dark green, and some hours afterwards to a dark brown or nearly black colour) was dissolved in nitric acid and the solution evaporated to dryness, alter which a red heat was applied in order to bring the iron to the state of peroxyd, and thus render it insoluble in the same acid. The mass being now treated with nitric acid, in order to separate the lime and magnesia supposed to be mixed with the oxyd of iron, and the whole being thrown into a filter, the clear solution was found still to contain a good deal of iron. This last solution was, like the former, evaporated to dryness, and to the residue, again heated to redness, acetic, instead of nitric acid, was this time added, and the solution filtered. The filtered fluid still contained a quantity of iron, which however, from subsequent examination, appeared very inconsiderable. The oxyd of iron left in the filter being roasted with wax and heated to redness, in order to bring it to an uniform state of oxidation, weighed 6,8 grs.
- By the word residue, thus generally used, is always meant the residue of the water under examination, dried at the temperature of between 170° and 180°. And in comparing a quantity of residue with a corresponding portion of the water, the average proportion of 80,5 grs. for each pint (§ VI. 2) is always assumed as the standard of comparison.
- The acetic acid, as well as the nitric, is said to be incapable of dissolving any Iron, which has been peroxydated by the process just described. In this instance a few particles of oxyd were taken up by the acid; but it is probable that if, instead of heating the residue to redness only for a few minutes, the oxyd had continued exposed to a red heat for half an hour or more, the whole of it would have become insoluble.
- It may be asked in what state of oxidation the iron is after that operation? It has generally been supposed to be reduced to the state of protoxyd in consequence of the affinity of the combustible matter for oxygen; but in an experiment which I made some years ago to ascertain that point, (the particulars of which my be seen in my account of the Brighton chalybeate) this process appeared to bring the iron to the state of peroxyd; for 100 parts of iron gave 147,6 parts of oxyd, proportions which are now considered as constituting the red oxyd of iron. And as a confirmation of this, I observe that Dr. Thomson in his valuable paper on the oxyds of iron, published in the twenty-seventh volume of Nicholson's Journal, states (page 379) that some of the red oxyd being mixed with oil and heated to redness till it became black and magnetic, no diminution of weight took place. Indeed I have always obtained by this process, not a black, but a brown oxyd, which in cooling passes to a red-brown colour, somewhat varying in shade, but mostly resembling powdered cinnamon, and being more or less magnetic.