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 orchil. 



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, Parmelia and Pertusaria, and above all Lecanora 

 tartarea (Fig. 134). The last named, one of the hardiest and most abundant 



Zopf 1907, p. 393. 



2 Lindsay 1855. 



