Volume 2

The elements of experimental chemistry / by William Henry.

  • Henry, William, 1774-1836.
Date:
1815
Catalogue details

Licence: Public Domain Mark

Credit: The elements of experimental chemistry / by William Henry. Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

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    mum, the ferro-prussiate of iron that is formed is of a white colour, but gradually becomes blue, as the iron, by exposure to air, passes to the state of peroxide *. (f) The effect of a sympathetic ink may be obtained, by writing with a pen dipped in a very dilute solution of ferro- prussiate of potash. No characters will appear till the paper is moistened with sulphate of iron, when letters of a Prussian blue colour will be apparent. The experiment may be re- versed, by writing with sulphate of iron, and rendering the characters legible by prussiate of potash. (g) The ferro-prussiate of potash decomposes all metalhc solutions, excepting those of gold, platina, iridium, osmium, rhodium, tellurium, and antimony f. . VI. When sulphate of iron is mixed with an infusion of galls, we obtain a black solution, which is a new combination of oxide of iron, with the gallic acid and tan. Both the gallatc and tannute of iron are, therefore, essential constitu- ents of inks; the other ingredients of which are chiefly added with theview of keeping these insoluble compounds suspended. In order that the iron may unite with the gallic acid and tan, it must be combined with the sulphuric acid in the state of red oxide; for the less oxydized iron, in the green salt, does not form a black compound with these substances. Iron filings, however, dissolve in an infusion of galls with an ex- trication of hydrogen gas; but the compound is not black till after exposure to air, which oxydizes the iron still farther. This solution, with a sufficient quantity of gum, forms an excellent ink. On the same principle may be explained the effect of me- tallic iron in destroying the colour of ink. When ink is digested with iron filings, and frequent]}' shaken, its colour decays; and it also becomes colourless after having a stream of sulphuretted hydrogen gas passed through it. In both these cases the oxide of iron is partly deoxydizcd. Characters written with ink, after this treatment, are at first illegible, .but become black as the iron acquires oxygen from the air. * See Proust's memoir, in Nicholson's Journal, t Sue Proust, Philosophical Magazine, .^xx, 4'^