CONCLUDING OBSERVATIONS. 169 



exist in water,— or, in other words, may be represented by carbon and 

 crater in various proportions. 



5°. That the food on which they live enters by the roots and leaves 

 of plants, — that the leaves, under the influence of the sun, decompose 

 the carbonic acid, give off its oxygen, and retain its carbon,— rand that 

 this carbon, uniting with the elements of water in the sap, forms those 

 several compounds of which plants chiefly consist. 



6°. That the supply of carbonic acid in the atmosphere is kept up 

 partly by the respiration of animals, partly by the natural decay of dead 

 vegetable matter, and partly by combustion. That ammonia is sup- 

 plied to plants chiefly by the natural decay of animal and vegetable 

 substances — and nitric acid partly by the natural oxidation of dead or- 

 ganic matter, and partly by the direct union of oxygen and nitrogen, 

 through the agency of the atmospheric electricity. 



7°. That while both of these compounds yield nitrogen to plants, they 

 each exhibit a special action on vegetable life, in virtue of the hydrogen 

 and oxygen they respectively contain — and exercise also a so-called 

 stimulating power, by which plants are induced or enabled to appro- 

 priate to themselves, from other natural sources, a larger portion of 

 all their constituent elements than they could otherwise obtain or 

 assimilate. 



In illustrating these several points, it has been necessary to enter oc- 

 casionally into details which, to those who have heard or may read only 

 the later lectures, may not be altogether intelligible. I am not aware, 

 however, of having introduced any thing of which the full sense will 

 not appear on a reference to the statement by which it is preceded. 



We are now to consider the inorganic constituents of plants, — their na- 

 ture, — the source (the soil) from which they are derived, — their uses in 

 the vegetable and animal economy, — how the supply of these substan- 

 ces is kept up in nature, — and how, in practical husbandry, the want of 

 them may be at once efficaciously and economically supjplied by art. 

 This division of our subject, though requiring a previous knowledge of the 

 principles discussed in the foregoing lectures, will be more essentially 

 of a practical nature, and will lead us to consider and illustrate the 

 great leading principle by which the practical agriculturist ought to be 

 guided in the cultivation and improvement of his land. 



We shall here also find much light thrown upon our path by the 

 results of geological inquiry ; and it is in the considerations I am now 

 about to bring before you, that I shall have to direct your attention most 

 especially to the principal applications of Geology to Agriculture. 



