AGRICULTURAL CHEMISTRY. 



31 



element by most of the ancient philoso- 

 phers: a few of the chemical enquirers in 

 the sixteenth and seventeenth centuries, 

 formed some happy conjectures respecting 

 its real nature. 



Sir Kenelm Digby in 1660, supposed 

 that it contained some saline matter, which 

 was an essential food of plants. Boyle, 

 Hooke, and Mayaw, between 1665 and 

 1680, stated that a small part of it only 

 was consumed in the respiration of ani- 

 mals, and in the combustion of inflammable 

 bodies; but the true statical analysis of the 

 atmosphere is comparatively a recent la- 

 bour, achieved towards the end of the 

 last century by Scheele, Priestly, and 

 Lavoisier. These celebrated men showed 

 that its principal elements are two gasses, 

 oxygen and azote, of which the first is 

 essential to flame, and lo the life of ani- 

 mals, and that it likewise contains small 

 quantities of aqueous vapour, and of car- 

 bonic acid gas; and Lavoisier proved 

 that this last body is itself a compound 

 elastic fluid, consisting of charcoal dissol- 

 ved in oxygen. 



Jethro Tull, in his treatise on Horse- 

 hoeing, published in 1733, advanced the 

 opinion that minute earthy particles sup- 

 plied the whole nourishment of the veg- 

 etable world; that air and water were 

 chiefly useful in producing these particles 

 from the land; and that manures acted 

 in no other way than in ameliorating the 

 texture of the soil, in short that, their 

 agency was mechanical. 



This ingenious author of the new sys- 

 tem of agriculture, having observed the 

 excellent efiects produced in farming by a 

 minute division of the soil, and the pul- 

 verization of it by exposure to dew and 

 air was misled by carrying his principle 

 too far. Duhamel, in a work printed in 

 1754, adopted the opinion of Tull, and 

 stated that by finely dividing the soil any 

 number of crops might be raised in suc- 

 cession from the same land. He attemp- 

 ted also to prove, by direct experiments, 

 that vegetables of every kind were capa- 

 ble of being raised without manure. This 

 celebrated horticulturist lived however 

 sufficiently long to alter his opinion. The 

 results of his later and most refined obser- 

 vations led him to the conclusion, that no 

 single material afforded the food of plants. 

 The general experience of farmers had 



long before convinced the unprejudiced of 

 the truth of the same opinion, and that 

 manures were absolutely consumed in 

 the process of vegetation. The exhaus- 

 tion of soils by carrying off' corn crops' 

 from them, and the efiects of feeding cat- 

 tle on lands, and of preserving their man- 

 ure, offer familiar illustrations of the 

 principle; and those philosophical enquir- 

 ers, particularly Hassenfratz and Saussure, 

 have shown by satisfactory experiments, 

 that animal and vegetable matters deposi- 

 ted in soils are absorbed by plants, and 

 become a part of their organized matter. 

 But though neither water, nor air, nor 

 earth, supplies the whole of the food of 

 plants, yet they all operate in the process 

 of vegetation. The soil is the laboratory 

 in which the food is prepared. No man- 

 ure can be taken up by the roots of plants 

 unless water is present; and water or its 

 elements exist in all the products of veg- 

 etation. The germination of seeds does 

 not take place without the presence of air 

 or oxygen gas; and in the sunshine veg- 

 etables decompose the carbonic acid ga^ 

 of the atmosphere, the caibon of which 

 is absorbed and becomes a part of their 

 organized matter, and the oxygen gas, 

 the other constituent, is given off; and in 

 consequence of a variety ot agencies, the 

 economy of vegetation is made subser- 

 vient to the general order of the sj^stem^ 

 of nature. 



It is shown by various researches that 

 the constitution of the atmosphere has 

 been always the same since the time that 

 it was first accurately analyzed; and this 

 must in a great measure depend upon the 

 powers of plants to absorb or decompose 

 the putrefying or decaying remains of an- 

 imals and vegetables, and the gaseous 

 effluvia which they are constantly emit- 

 ing. Carbonic acid gas is formed in a 

 variety of processes of fermentation and 

 combustion, and the respiration of ani- 

 mals, and as yet no other process is 

 known in nature by which it can be con- 

 sumed, except vegetation. Animals pro- 

 duce a substance which appears to be a 

 necessary food for vegetables; vegetables 

 evolve a principle necessary to the exis- 

 tence of animals; and these different clas- 

 ses of beings seem to be thus connected 

 together in the exercise of their living 

 functions, and to a certain extent made to 



