. Lichens. Lichens. 414 ECONOMIC AND TECHNICAL An aqueous solution of litmus when exactly neutralized by an acid is violet coloured; it becomes red with the smallest trace of free acid, or blue with free alkali. Litmus paper is prepared by steeping specially prepared unsized paper in the dye solution. It is as a ready and sensitive indicator of acidity or alkalinity that litmus is of so much value. According to Zopf' it is also used as a blueing agent in washing and as a colouring of wine. Litmus is chiefly manufactured in Holland. Still another substance some- what differently prepared from t
. Lichens. Lichens. 414 ECONOMIC AND TECHNICAL An aqueous solution of litmus when exactly neutralized by an acid is violet coloured; it becomes red with the smallest trace of free acid, or blue with free alkali. Litmus paper is prepared by steeping specially prepared unsized paper in the dye solution. It is as a ready and sensitive indicator of acidity or alkalinity that litmus is of so much value. According to Zopf' it is also used as a blueing agent in washing and as a colouring of wine. Litmus is chiefly manufactured in Holland. Still another substance some- what differently prepared from the same lichens is sold as French purple, a more brilliant and durable colour than ^1 Fig. 134. Lecanora tartarea Ach. (S. H., Photo). d. Other Orchil Lichens. Though species of Roccella rank first in importance as dye-plants, purple and blue colours are obtained, as indicated above, from other very different lichens. Lindsay' extracted orchil from about twenty species. Those most in use in northern countries are on the whole less rich in colouring substances; they are: Umbilicaria pustulata, species of Gyrophora, Parnielia and Pertusaria, and above all Lecanora tartarea (Fig. 134). The last named, one of the hardiest and most abundant 1 Zopf 1907, p. 393. ^ Lindsay Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Smith, Annie Lorrain. Cambridge, At the University Press
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