CHAP. II. FUNCTION OF NUTRITION. IQ1 



also known to be eminently beneficial to them. These 

 requisites will at once exclude a large class of com- 

 pounds, to be met with only in certain families of plants, 

 as well as several others which are known to exercise 

 noxious effects upon vegetation. And thus we find, 

 upon careful inquiry, that our choice is restricted to 

 about four substances, all of which possess nearly the 

 same chemical characters, and which are the most uni- 

 versally present among the juices of plants. These are 

 gum, sugar, fecula, and lignine. The first of these ap- 

 pears by far the most universally diffused, and has been 

 obtained from nearly every plant in which it has been 

 sought for ; and moreover as it possesses decidedly nutri- 

 tious qualities, it may be considered with every proba- 

 bility in its favour, as the first or proximate organisable 

 compound formed by the action of vegetable life, acting 

 under the stimulus of light. The other three substances, 

 which so nearly resemble gum in chemical composition, 

 appear to be slight modifications of it, which have re- 

 sulted from some further elaborations perfected by the 

 vesicles in different parts of the vegetable structure, and 

 we shall defer their description to our account of the 

 sixth period of nutrition. 



(177.) Gum exudes naturally from certain trees, 

 and especially from some acacias, which furnish the 

 common gum-arabic of commerce. It is purer when 

 obtained in this way than when it has been separated 

 by some chemical process from the sap. Its specific 

 gravity varies from 1-316 to 1-482. It is extremely 

 soluble in water, but is insoluble in alcohol, ether, and 

 and oil. It possesses slight modifications in its qua- 

 lities, according as it is extracted from different plants ; 

 and the following analysis will show its composition, as 

 it has been stated by three eminent chemists : 



