COMPOSITION OF PLANTS. 



221 



344. 



The third peculiarity of the composition of organic bodies^ 

 respects the absence, commonly, in such bodies, of any of the 

 ordinary chemical ingredients in an entirely disengaged state. 

 Completely disengaged acids are as seldom to be met with in 

 the vegetable as in the animal juices. For the most part 

 they are united to a base, and are first disengaged during fer- 

 mentation, by the influence of mineral acids, or by some other 

 operations. It is equally seldom that we find, in a free state, 

 in the vegetable kingdom, any of those ingredients of which 

 hydrogen is a part. These, like alcohol, are first disengaged, in 

 consequence of a change which takes place during the saccha- 

 rine fermentation. It is true, that, in a few instances, free acids 

 are found in plants, and that volatile oils are given out by them. 

 But these matters are for the most part to be considered as 

 excrementitious, as the oxalic acid which is exhaled from the 

 Chick-pea ; besides, the hydrogen, in volatile oils, is too close- 

 ly united to other matters, to be regarded as an entirely free 

 body, 



345. 



The fourth peculiarity in the composition of organic bo- 

 dies, consists in a kind of circulation, which the simple con^ 

 nections of the elementary bodies in the sustaining juices un- 

 dergo. As in the higher animals, the chyle by degrees be- 

 comes freed from its oxygen, while it is mingling with the 

 gall and passing through the glands of the intestines, that at 

 last, in the thoracic duct it may pass into the state of blood 

 by the union of azote, by the evolution of phosphorated iron, 

 and of the colouring materials ; as this neutral fluid frees 

 itself in the secreting organs from its hydrogen, azote, 

 oxygen, and carbon, that it may suffer a new oxydation in 

 the lungs, and be prepared for undergoing again the same 

 changes; — in the sam.e manner plants attract carbonic acid 

 water saturated with azote ; mix it with their own substance, 

 and sometimes add more evolved hydrogen and carbon to the 

 oxygen ; and at other times free themselves from their super- 

 fluous oxygen and carbonic acid by exhalation from the 



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