CHANGES PRODUCED BY NOURISHMENT. 99 



l^ained during the progress of change in the roots, is want- 

 ing for the formation of nitric acid ; this, combined with 

 the potash, which is contained in these roots in the pro- 

 portion of Y^^ of its weight, would produce salt-petre. 



If we observe a plant during the various stages of its 

 vegetation, we shall perceive at these different periods very 

 remarkable differences in the odor, taste, consistency, 

 &c. ; from this circumstance we must suppose that it forms 

 new- products, new combinations, and consequently new 

 salts. 



Th^ alkaline salts are the most abundant in green her- 

 baceous plants. M. de Saussure has observed, that the 

 ashes of young plants that grew upon a poor soil contained 

 at least f of their weight of alkaline salts, and that those 

 of leaves of trees which grew from their buds contained 

 at least ^, 



The proportion of alkaline salts diminishes in propor- 

 tion as the plants advance in age : this remark applies 

 equally to annual plants and to the leaves of those trees 

 that shed their foliage in autumn. The ashes of seeds 

 contain a greater proportion of alkaline salts, than those 

 of the plants that produced them. 



These facts are very important to those who are engaged 

 in the manufacture of salts furnished by the combustion 

 of vegetaWe substances; since they show clearly that it 

 cannot be equally advantageous to them to consume all 

 sorts of plants, nor at all periods of their growth. 



Next to the alkaline salts, the earthy phosphates of lime 

 and magnesia are the most abundant in plants, and, like 

 the first, these diminish in quantity in proportion to the age 

 of the plant. Plants also contain, but in very small pro- 

 portions, silica, and some metallic oxides, especially those 

 of iron. 



ARTICLE VL 



TAe Changes produced in Plants by Nourishment^ resumed. 



Plants are principally nourished through their leaves 

 and roots : the first absorb from the atmosphere oxygen, 

 carbonic acid, and water j and the second receive from 



