218 



HERBS 



has, when chewed, a mucilaginous, bitter taste. A decoction (1 to 20) 

 yields on cooling a jelly which is stained blue by iodine 



Constituents. The principal constituent of Iceland moss is the 

 carbohydrate lichenin, C 6 H 10 O 5 , which is accompanied by isolichenin 

 (dextrolichenin, Fliickiger). Isolichenin is soluble in cold water, and 

 behaves as a soluble modification of starch. Lichenin dissolves in 

 boiling water, but the solution gelatinises on cooling. Both lichenin 

 and isolichenin are converted by hydrolysis with dilute mineral 

 acids into dextrose ; as much as 72 per cent, of fermentable sugar 

 can by this means be obtained from the drug. 



FIG. 1 14. Iceland Moss (Cetraria islandica. ) Natural size. (Luerssen. ) 



Iceland moss also contains cetraric acid, C 20 H 18 9 , a crystalline 

 bitter substance, almost insoluble in water, but forming soluble acid 

 salts with monovalent alkalies ; hence the bitterness of the drug can 

 be removed by soaking in dilute solution of sodium bicarbonate. 

 Other constituents of the drug are bitter protocetraric acid and 

 tasteless lichenostearic acid. 



5. The properties of Iceland moss are those of a bitter tonic 

 and nutritive, but it is now seldom employed. 



Note. Litmus is a colouring matter obtained from various lichens, chiefly 

 Roccdla tinctoria, de Candolle (Cape Verde). E. Montagnei, Bel. (Madagascar), 

 Dendrographa leucophcea, Darbish, &c. (N.O. Discomycetes). The method of 

 preparation is guarded as a trade secret, but it appears to depend mainly upon 



