Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V22, Page 693 of the 1911 Encyclopedia Britannica.
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PYROGALLOL, or PYROGALLIC ACID, a trioxybenzene, C6H3(OH)3 (1 : 2 : 3), prepared by Scheele in 1786 by heating gallic acid, C6H2(OH)3CO2H. It is also obtained by heating para-chlorphenoldisulphonic acid with potassium hydroxide. It forms white plates, melting at 132°, readily soluble in water, and subliming without decomposition. It is an energetic reducing agent, a property utilized in its application in gas analysis to absorb oxygen, and in photography (q.v.) as a developer. The aqueous solution is turned bluish black by ferrous sulphate containing a ferric salt. It does not combine with hydroxylamine, as does the isomeric phloroglucin which yields a trioxime(see POLYMETnYLENES). Pyrogallol dimethyl ether is found in beechwood tar. Pyrogallol has antiseptic properties and is employed medicinally in the treatment of psoriasis. Eugallol, or monacetyl pyrogallol and lenigallel, or triacetyl pyrogallol, are also used. position of various rocks, for example, limestone. That it is readily deposited from solution is shown by the frequent occurrence of black dendritic markings in the crevices of rocks, excellent examples of which are seen in mocha stone (q.v.) and in the lithographic stone of Solenhofen in Bavaria. It is deposited from the waters of some springs, and manganiferous nodules are dredged from the floor of the deep sea. As an ore it is extensively mined at Ilmenau and several other places in Thuringia, at Vorderehrensdorf near Prossnitz in Moravia, Platten in Bohemia, in North Wales, at several places in the United States (Vermont, Virginia, Arkansas, &c.), Nova Scotia and Brazil. Pyrolusite, together with the rather less important ore, psilomelane, has various economic applications. It is extensively used for the manufacture of spiegeleisen and ferromanganese, and of various alloys, such as manganese-bronze. As an oxidizing agent it is used in the preparation of chlorine and disinfectants (permanganates), and for decolorizing glass: when mixed with molten glass it oxidizes the ferrous iron to ferric iron, and so discharges the green and brown tints, hence the name pyrolusite, from Gr. 7-up (fire) and ab€iv (to wash). As a colouring material, it is used in calico printing and dyeing; for imparting violet, amber and black colours to glass, pottery and bricks; and in the manufacture of green and violet paints. (L. J. S.)
PYROMETER (Gr. iri p, fire, µErpov, a measure)

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