ON THE FOOD OF PLANTS. 119 



" principles necessary for the life of plants j but 

 " there are few cases in which they can be ap- 

 " plied as manures in their pure forms, and 

 " vegetable manures in general contain a great 

 " excess of fibrous and insoluble matter, which 

 " must undergo chemical changes before they 

 " become the food of plants." 



I must confess I do not see the utility of Sir 

 H. Davy's expressing such an opinion. To say 

 the best of it, it appears, as Mr. Kirwan observes, 

 to be but a speculation ; and as he does not state 

 the few cases, it is worse than useless, inasmuch 

 as it is calculated to mislead, bewilder, and con- 

 fuse, and to shake the confidence of mere prac- 

 tical husbandmen in the general doctrines of 

 chemistry. 



I cannot but think it by far the most probable, 

 that Sir Humphry Davy's first idea was the 

 correct one ; and that such substances must be 

 reduced by fermentation before they can be taken 

 up by the roots. At any rate, under those cir- 

 cumstances, the effect of such an application as 

 he describes, to the roots of the plants, must 

 have been the same, that is, death : but another 

 cause of death much more probable may be as- 

 signed, than that of the vessels being clogged 

 with solid matter; viz. the oxygenization, or 

 acidifying of those solutions, which most likely 



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