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CHAPTER VI. 



VEGETABLE PRINCIPLES — NATURE AND COMPOSITION 

 OF ANIMAL SUBSTANCES. 



491. It is well known that plants possess very 

 various properties : some are noted for their fragrant 

 smell, others for the brilliant colors they produce ; 

 some yield oils, others resins, and a great many are 

 valued for their peculiar medicinal qualities : all 

 these various properties are caused by the presence 

 of a certain quantity of some organic substance, some 

 peculiar compound of oxygen, hydrogen, carbon, 

 and nitrogen, which is formed by the plant. 



492. The number of these organic substances 

 which have been discovered in plants is very great : 

 it is quite unnecessary to describe or even enumerate 

 them all ; it will be sufficient to mention very briefly 

 the most important of them, arranged under three or 

 four heads. 



493. Amongst the substances formed by plants, is 

 a numerous series of organic acids : substances com- 

 posed of oxygen, hydrogen, and carbon, which pos- 

 sess acid powers, and combine eagerly with bases to 

 form neutral salts. In general, they do not exist in 



