Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/238

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Although I found it expedient, before advancing farther in the examination of the water, and in order to regulate my steps in the progress of the inquiry, thus to trace the various plans which seemed adapted to the purpose, yet I apprehend it would be superfluous to detail here in regular succession all the trials which arose from these different methods. I shall therefore confine myself to such as belong more immediately to my object; and in relating them, shall consider singly and under separate heads, the various ingredients of the water, stating, as I proceed, the proportions in which they were ultimately obtained.


§ VIII. Sulphat of Iron.

The presence of iron, in the state of sulphat, having been abundantly proved by the preliminary experiments, the next step was, to ascertain the proportion of this salt in a given quantity of the water. The first reagent which I tried for this purpose, was prussiat of potash; but after many trials which afforded uncertain and discordant results, I convinced myself that this test, however useful for detecting the presence of iron, is quite inappropriate when our object is to ascertain the quantity of that substance.[1]

  1. Prussiat of potash, as a precipitant of iron, is liable to the following objections.

    1st. It is apt, although apparently well prepared and crystallized, to precipitate certain earthy substances, and in particular alumine; this I found distinctly to happen in two experiments in which the mixture was heated.

    2dly. If the solutions be used cold, and if the metal be not highly oxydated, some of the Prussian blue unavoidably passes through the filters; or if no filters be used, it subsides but slowly and imperfectly.

    3dly. If the solutions be heated, the prussiat of potash is itself decomposed, and yields quantity of oxyd of iron which vitiates the results.