LI CHEN ALES. 681 



acted upon by ammonia. These resins have been examined by Dr. Kane 

 and other chemists, and have received a variety of names. 



The blue dye known under the name of Archil or Orchil, is principally 

 prepared from Rocella tinctoria, but other kinds of Lichen are also used. 

 Litmus or Lacmus, and Cudbear, are prepared from Lecanora tartarea, L. 

 parellus, &c. This article is of a blue colour, and is employed as a dye, 

 and also as a test for acids and alkalies ; the former changing its solution to 

 a red, and the latter restoring the original blue colour, when thus reddened. 

 Numerous other species might be mentioned as producing dyes, but it may 

 be sufficient to say, that all the crustaceous kinds are useful, and many of the 

 foliaceous. 



