84 



POPULAR HISTORY OJb 1 LICHENS. 



Umhilicarias. Their other properties are due to the pre- 

 sence of bitter principles, such as cetraric acid, — or astringent 

 substances, such as gallic or tannic acid. Hence the em- 

 ployment of the common Parmelia parietina, Physcia fur- 

 furacea and prunastri, some Cladonias, and the variolarioid 

 forms of Pertusaria communis, as febrifuges in intermittent 

 and other fevers, or as astringents in various intestinal and 

 chest affections ; of Cetraria Islandica and Sticta pulmonaria 

 as tonics ; or of various Umhilicarias and Peltigeras as pur- 

 gatives or vermifuges. 



But the most important economical application of the 

 Lichens is based on the valuable purple dyes which many 

 species are capable of yielding. These are producible, and 

 are usually produced in the course of manufacture, whether 

 on the large or small scale, by the joint action of atmo- 

 spheric an\ water, and ammonia on certain colourless, nitro- 

 genous, organic acids, which, from the names of the genera 

 in which they have been first or chiefly found, have been 

 denominated by their discoverers Orcellic, Lecanoric, Gyro- 

 plioric, Euernic acids, etc. The metamorphosis of colour 

 appears to take place, in connection with certain alkalies, 

 by a process of oxidation ; but we cannot here enter on the 

 chemistry of the change or of its products. This subject, we 



