VI. VEGETABLE SUBSTANCES BUT LITTLE 

 KNOWN. 



THERE is a large number of compounds occurring in 

 nature, whose chemical constitution and the relation 

 they bear to other better known bodies have not yet 

 been ascertained. Only the more important and better 

 investigated of these will be here described. 



A. ACIDS. 



1. Usnic acid, C 18 H 18 7 . In a great many lichens, 

 particularly in the various species of Usnea, from 

 which it is extracted by means of ether. Sulphur- 

 yellow, transparent prisms; insoluble in water, but 

 sparingly soluble in alcohol ; fusible at 202. (A modi- 

 fication of usnic acid, from Cladonia rangiferina, called 

 beta-usnic acid, fuses at 175). Its solution, in an excess 

 of alkali, becomes first red and then black in the air. 

 Subjected to dry distillation, it yields betaorcin (p. 309). 



2. Cetraric acid, C 18 II 15 8 . In Iceland moss (Cetraria 

 islandicd). It can be obtained pure only with diffi- 

 culty. Very fine, white needles, of an intensely bitter 

 taste ; neither fusible nor volatile ; scarcely soluble in 

 water, easily soluble in alcohol. Dissolves in alkalies 

 with yellow color, which is, however, rapidly converted 

 into brown in the air, the acid undergoing decomposi- 

 tion. It suifers a similar rapid decomposition when 

 boiled in alcohol or water, with access of air. 



