ABSORPTION OF SALTS. 59 



while those left by an oak from the department of la Somme, contain- 

 ed much magnesia and phosphate of lime.* The ashes from a white 

 mulberry of Nemours contained more than 0,10 of phosphoric acid, 

 while scarcely any traces of it were found in those of a similar mul- 

 berry from the calcareous soil of Provence. The most remarkable 

 inference deducible from the analyses of M. Berthier, is that which 

 is connected with the composition of the ashes yielded by trees 

 growing in the same soil. It is observed that, for analogous species, 

 the ashes also bear the closest analogy ; and on the contrary, it is 

 found that trees of very distinct genera yield ashes of quite a dif- 

 ferent quality ; results which lead to this important conclusion, that 

 plants possess the faculty of selecting in the soil the substances 

 which are best suited to their special organizations. This is a point 

 which we shall have an opportunity of discussing, when we come 

 to treat of rotations of crops. 



The substances composing the ashes of vegetables, are not all in 

 the state in which they existed in the vegetable tissue. In plants 

 there constantly exist organic acids, which, in general, are combined 

 with mineral bases. During the incineration of plants these organic 

 acids are destroyed, and the result of their destruction is an alkaline 

 carbonate, if the pre-existing acid was united with soda or potash ; 

 a calcareous vegetable salt, again, yields carbonate of lime ; and a 

 magnesian salt gives magnesia, from the well-known inability of this 

 earth to retain carbonic acid at a high temperature. Thus, the 

 greater part of the carbonates which enter into the composition of 

 vegetable ashes, are formed by the mere fact of incineration. The 

 salts which resist the action of a strong heat, as the phosphates, sul- 

 phates, and chlorides, are the only ones which in the ashes retain 

 the state in which they existed in the living plant. 



Water being the vehicle which must convey the mineral salts from 

 the soil into the vegetable, we do not always perceive how they can 

 penetrate. To explain the presence in plants of a salt so insoluble 

 as the neutral phosphate of lime, M. de Saussure admits, from satis- 

 factory experiments, that vegetable juices contain the double phos- 

 phate of potash and lime, and of potash and magnesia, f Besides, 

 several bodies considered in chemistry as insoluble, are not so abso- 

 lutely. Silica seems to possess a certain degree of solubility, — at 

 least, M. Payen has met with it in considerable quantity in the wa- 

 ter of the Artesian well of Grenelle, and in the water of the Seine. 

 We know% moreover, that several insoluble earthy salts are dissolv- 

 ed in virtue of the carbonic acid always contained in the waters 

 with which the soil is soaked. Lastly, it is not improbable that 

 certain insoluble salts have their origin in the plant itself, engender- 

 ed there by the successive arrival and reciprocal action of soluble 

 salts. 



It now remains for us to examine by what means saline substances 

 are introduced into the tissues of vegetables, and within what limits 



* These ashes were from the carbon of the oak ; the insoluble part gave 0.14 ol 

 phosphate of lime, and 0.08 of magnesia. 



t Saussure, Recherches chimiques, &c. p, 321. 



