FAMILIAR LETTERS ON CHEMISTRY. 



coriaceous or fleshy leaves with great force. They lose very little by evaporation, 

 compared with other plants. On the other hand, how very small is the quantity of 

 mineral substances which they withdraw from the soil during their almost constant 

 growth in one year, in comparison with the quantity which one crop of wheat of an 

 equal weight receives in three months ! 



It is by means of moisture that plants receive the necessary alkalies and salts 

 from the soil. In dry summers a phenomenon is observed, which, when the import- 

 ance of mineral elements to the life of a plant was unknown, could not be 

 explained. The leaves of plants first developed and perfected, and, therefore, 

 nearer the surface of the soil, shrivel up and become yellow, lose their vitality, and 

 fall off while the plant is in an active state of growth, without any visible cause. 

 This phenomenon is not seen in moist years, nor in evergreen plants, and but 

 rarely in plants which have long and deep roots, nor is it seen in perennials in 

 autumn and winter. 



The cause of this premature decay is now obvious. The perfectly developed 

 leaves absorb continually carbonic acid and ammonia from the atmosphere, which 

 are converted into elements of new leaves, buds, and shoots ; but this metamorphosis 

 cannot be effected without the aid of the alkalies, and other mineral substances. If 

 the soil is moist, the latter are continually supplied to an adequate amount, and the 

 plant retains its lively green color; but if this supply ceases from a want of 

 moisture to dissolve the mineral elements, a separation takes place in the plant 

 jtself. The mineral constituents of the juice are withdrawn from the leaves 

 already formed, and are used for the formation of the young shoots ; and as soon 

 as the seeds are developed, the vitality of the leaves completely ceases. These 

 withered leaves contaiu only minute traces of soluble salts, while the buds and 

 ghoots are very rich in them. 



On the other hand, it has been observed, that where a soil is too highly impreg- 

 nated with soluble saline materials, these are separated upon the surface of the 

 leaves. This happens to culinary vegetables especially, whose leaves become 

 covered with a white crust. In consequence of these exudations the plant sickens, 

 its organic activity decreases, its growth is disturbed ; and if this state continues 

 long, the plant dies. This is most.frequently seen in foliaceous plants, the large 

 surfaces of which evaporate considerable quantities of water. Carrots, pumpkins, 

 peas, &c., are frequently thus diseased, when, after dry weather, the plant being 

 near its full growth, the soil is moistened by short showers, followed again by dry 

 weather. The rapid evaporation carries off the water absorbed by the root, and this 

 leaves the salts in the plant in a far greater quantity than it can assimilate. These 

 salts effloresce upon the surface of the leaves, and if they are herbaceous and juicy, 

 produce an effect upon them as if they had been watered with a solution containing 

 a greater quantity of salts than their organism can bear. 



Of two plants of the same species, this disease befalls that which is nearest its 

 perfection ; if one should have been planted later, or be more backward in its 

 development, the same external cause which destroys the one will contribute to the 

 growth of the other. 



LETTER XII. 



MY DEAR SIR: 



Having now occupied several letters with the attempt to unravel, by means of 

 chemistry, some of the most curious functions of the animal body, and, as I hope, 

 made clear to yeu the distinctions between the two kinds of constituent elements 

 in food, and the purposes they severally subserve in sustaining life, let me now 

 direct your attention to a scarcely less interesting and equally important subject 

 the means of obtaining from a given surface of the earth the largest amount of 

 produce adapted to the food of man and animals. 



Agriculture is both a science and an art. The knowledge of all the conditions of 

 the life of vegetables, the origin of their elements, and the sources of their nourish- 

 ment, form its scientific basis. 



From this knowledge we derive certain rules for the exercise of the ART, the 

 principles upon which the mechanical operations of farming depend, the usefulness 

 or necessity of these for preparing the soil to support the growth of plants, and for 



