FAMILIAR LETTERS ON CHEMISTRY. 4} 



however, the ashes of food contain no alkaline phosphates, but abound in insoluble 

 earthy phosphates, as hay, carrots, and potatoes, the urine will, be free from alka- 

 line phosphates, but the earthy phosphates will be found in the faeces. The urine 

 of man, of carnivorous and graminivorous animals, contains alkaline phosphates; 

 that of herbivorous animals is free from the salts. 



The analysis of the excrements of man, of the piscivorous birds (as the guano,) 

 of the horse, and of cattle, furnishes us with the precise knowledge of the salts 

 they contain, and demonstrates, that in those excrements, we return to the fields 

 the ashes of the plants which have served as food the soluble and insoluble salts 

 and earths indispensable to the development of cultivated plants, and which must 

 be furnished to them by a fertile soil. 



There can be no doubt that, in supplying these excrements to the soil, we return 

 to it those constituents which the crops have removed from it, and we renew ita 

 capability of nourishing new crops : in one word, we restore the disturbed equili- 

 brium ; and, consequently, knowing that the elements of the food derived from the 

 soil enter into the urine and solid excrements of the animals it nourishes, we can, 

 with the greatest facility, determine the exact value of the different kinds of ma- 

 nure. Thus, the excrements of pigs, which we have fed with peas and potatoes, 

 are principally suited for manuring crops of potatoes and peas. In feeding a cow 

 upon hay and turnips, we obtain a manure containing the inorganic elements of 

 grasses and turnips, and which is, therefore, preferable for manuring turnips. The 

 excrement of pigeons contains the mineral elements of grain; that of rabbits, the 

 elements of herbs and kitchen vegetables. The fluid and solid excrements of man, 

 however, contain the mineral elements of grain and seeds in the greatest quantity. 



LETTER XV. 



MY DEAR SIR: 



You are now acquainted with my opinions respecting the effects of the appli- 

 cation of mineral agents to our cultivated fields, arid also the rationale of the 

 influence of the various kinds of manure; you will, therefore, now readily under- 

 stand what I have further to say of the sources whence the carbon and nitrogen, 

 indispensable to the growth of plants, are derived. 



The growth of forests, and the produce of meadows, demonstrate that an inex- 

 haustible quantity of carbon is furnished for vegetation by the carbonic acid of 

 the atmosphere. 



We obtain from an equal surface of forest, or meadow-land, where the necessary 

 mineral elements of the soil are present in a suitable state, and to which no carbo- 

 naceous matter whatever is furnished in manures, an amount of carbon, in the 

 shape of wood arid hay, quite equal, and ofttimes more than is produced by our 

 fields, in grain, roots, and straw, upon which abundance of manure has been 

 heaped. 



It is perfectly obvious that the atmosphere must furnish to our cultivated fields 

 as much carbonic acid, as it does to an equal surface of forest or meadow, and that 

 the carbon of this carbonic acid is assimilated, or may be assimilated by the 

 plants growing there, provided the conditions essential to its assimilation, and be- 

 coming a constituent element of vegetables, exist in the soil of these fields. 



In many tropical countries the produce of the land in grain or roots, during the 

 whole year, depends upon one rain in the spring. If this rain is deficient in quan- 

 tity, or altogether wanting, the expectation of an abundant harvest is diminished 

 01 destroyed. 



Now it cannot be the water merely which produces this enlivening and fertilizing 

 effect observed, and which lasts for weeks and months. The plant receives, by means 

 of this water, at the time of its first development, the alkalies, alkaline earths, and 

 phosphates necessary to its organization. If these elements, which are necessary 

 previous to its assimilation of atmospheric nourishment, be absent, its growth is 

 retarded. In fact, the development of a plant is in a direct ratio to the amount of 

 the matters it takes up from the soil. If, therefore, a soil is deficient in these 

 mineral constituents, required by plants, they will not flourish even with an abun- 

 dant supply of water. 



The produce of carbon on a meadow, or an equal surface of forest land, is inde- 



