AGRICULTURE AND PLANTING. 17 



On the component Principles of Vegetables. 



tamarinds, sorrel, Sec. 



The mucilage called gum arable is obtained frpm in^mosa nilQtica, gunr 

 tragacanth exudes from astragalus tragacantka, as a similar gum exudes from 

 our cherry and plumb-trees; sagoe is the pith of the lycas circinalis ; and salep 

 is '"'.iC root of the ore/?/'* dried in an oven. 



This mucilage seems to serve as nourishment to the plant; first, because' it 

 is found in all vegetable as well as animal materials, as they decompose in dung- 

 hills; secondly, because it forwards the growth of vegetables, when spread upon 

 land; thirdly, because those trees, which bleed much gum, are weakened and fre- . 

 quently die; and lastly, because it is evidently laid up in the roots and seeds of 

 various vegetables for the nourishment of the young plants. But in these it seems 

 to undergo a change either in part chemical, or wholly by the digestive organs of 

 the embryon plant, and is converted into sugar, as in the transmutation of barley 

 into malt; and as appears from the sweet taste of onions and potatoes, when boil- 

 ed after they have germinated ; and as sugar abounds in the vernal sap-juice of 

 trees in such quantity as to be capable of fermentation. 



II. Concerning Oils. 



By common consent the name of oil is given to fat unctuous substances, 

 more or less fluid, insoluble in water, and combustible. 



These products appear to belong exclusively to animals and vegetables. 

 The mineral kingdom exhibits only a few substances of this nature, which possess 

 scarcely any of the above properties, such as the unctuous property. 



Oils are distinguished, relative to their fixity, into fat oils, and essential 

 oils. I shall describe them in ttiis article under the names of fixed oils and 

 volatile oils. 



1. Concerning fixed Oils. 



MosTof the fixed oils are fluid; but the greater number ^re capable of pass- 

 ing to the state of solidity, even by a moderate degree of cold. There are some 



