44 VEGETABLE DRUGS WITHOUT ORGANIC STRUCTURE. 



oils and oil of bitter almonds, cherry laurel, and wild 

 cherry bark. In wild cherry bark, cherry laurel, the 

 nitrile compounds form the basis of hydrocyanic acid 

 (HCN), which is the nitrile of formic acid, HCOH(N), 

 whereas the sulphur compounds found in mustard oils 

 make the volatile oil of mustard and impart to it its 

 rubefacient and stimulating qualities. 



It can be seen, therefore, from the consideration of 

 the chemistry of this class of bodies, that whereas terpenes 

 are to be found in all of them, the presence of more active 

 compounds, such as the ketones, the phenols, nitrile 

 compounds, and thiocyanates, overshadows the weaker 

 terpenes physiologically. This emphasizes the necessity 

 of making a provisional classification of the volatile 

 oils along chemical rather than botanical lines. A de- 

 tailed consideration of the various classes from the 

 physiological point of view belongs properly to the sub- 

 ject of pharmacology. 



RESINS. 



The resins as a class are difficult of definition. They 

 play an important role in pharmacy and medicine, how- 

 ever, and a knowledge of their composition is impera- 

 tive. Like the oils, resins are not definite compounds, but 

 mixtures of chemical substances. In their general gross 

 characters they closely resemble gums in that they are 

 hard and more or less tenacious. They are insoluble in 

 water — a point to be remembered as a feature of chemical 

 incompatibility. For the most part resins are soluble 

 in ether, in alcohol, and in carbon disulphide. They 

 are rich in carbon, poor in oxygen, and lacking in 

 nitrogen, and burn with a sooty flame. No resins are 

 chemical entities. The resin that flows from a wound 

 in a resiniferous plant to-day may differ slightly in the 

 percentage of its chemical constituents from that which 

 flows to-morrow, and the resin derived from one part of 



