AGRICULTURAL CHEMISTRY. 



must be some means of replacing the oxy- 

 gen, which is removed from the air by 

 the processes of combustion and putre- 

 faction, as well as by the respiration of 

 anmials. 



Both these causes are united in the pro- 

 cess of vegetable life. 



The facts which we have stated in the 

 preceding pages prove that the carbon of 

 plants must be derived exclusively from the 

 atmosphere. Now, carbon exists in the 

 atmosphere only in the form of carbonic 

 acid, and therefore, in a state of combination 

 with oxygen. 



It has been already mentioned likewise, 

 that carbon and the elements of water form 

 the principal constituents of vegetables; the 

 quantity of the substances which do not 

 possess this composition being in a very 

 small proportion. Now, the relative quan- 

 tity of oxygen in the whole mass is less than 

 in carbonic acid ; for the latter contains two 

 equivalents of oxygen, while one only is 

 required to unite with hydrogen in the pro- 

 portion to form water. The vegetable pro- 

 ducts which contain oxygen in larger pro- 

 portion than this, are, comparatively, few in 

 number; indeed, in many the hydrogen is in 

 great excess. It is obvious, that when the 

 hydrogen of water is assimilated by a plant, 

 the oxygen in combination with it must be 

 liberated, and will afford a quantity of this 

 t lament sufficient for the wants of the plant, 

 li' tins be the case, the oxygen contained in 

 the carbonic acid is quite unnecessary in the 

 process of vegetable nutrition, and it will 

 consequently escape into the atmosphere in 

 a gaseous form. It is, therefore, certain, that 

 plants must possess the power of decom- 

 posing carbonic acid, since they appropriate 

 its carbon for their own use. The forma- 

 tion of their principal component substances 

 must necessarily be attended with the sepa- 

 ration of the carbon of the carbonic acid 

 from the oxygen, which must be returned to 

 the atmosphere, while the carbon enters 

 into combination with water or its elements. 

 The atmosphere must thus receive a volume 

 of oxygen for every volume of carbonic 

 acid which has been decomposed. 



This remarkable property of plants has 

 been demonstrated in the most certain man- 

 ner, and it is in the power of every person 

 to convince himself of its existence. The 

 leaves and other green parts of a plant ab- 

 sorb carbonic acid, and emit an equal 

 volume of oxygen. They possess this pro- 

 perty quite independently of the plant; for if, 

 after being separated from the stem, they are 

 placed in water containing carbonic acid, 

 and exposed in that condition to the sun's 

 light, the carbonic acid is, after a time, 

 found to have disappeared entirely from the 

 water. If the experiment is conducted un- 

 der a glass receiver filled with water, the 

 oxygen emitted from the plant may be col- 

 ected and examined. When no more oxy- 

 ,gen gas is evolved, it is a sign that all the 

 dissolved carbonic acid is decomposed ; but 



the operation recommences if a new portion 

 of it is added. 



Plants do not emit gas when placed in 

 water which either is free from carbonic 

 acid, or contains an alkali that protects it 

 from assimilation. 



These observations were first made by 

 Priestly and Sennebier. The excellent ex- 

 periments of De Saussure have farther 

 shown, that plants increase in weight dur- 

 ing the decomposition of carbonic acid and 

 separation of oxygen. This increase in 

 weight is greater than can be accounted for 

 by the quantity of carbon assimilated ; a fact 

 which confirms the view, that the elements 

 of water are assimilated at the same time. 



The life of plants is closely connected 

 with that of animals, in a most simple man- 

 ner, and for a wise and sublime purpose. 



The presence of a rich and luxuriant vege- 

 tation may be conceived without the con- 

 currence of animal life, but the existence of 

 animals is undoubtedly dependent upon the 

 life and development of plants. 



Plants not only afford the means of nutri- 

 tion for the growth and continuance of ani- 

 mal organization, but they likewise furnish 

 that which is essential for the support of the 

 important vital process of respiration ; for, 

 besides separating all noxious matters from 

 the atmosphere, they are an inexhaustible 

 source of pure oxygen, which supplies the 

 loss which the air is constantly sustaining. 

 Animals on the other hand expire carbon, 

 which plants inspire; and thus the compo- 

 sition of the medium in which both exist, 

 namely, the atmosphere, is maintained con- 

 stantly unchanged. 



It may be asked is the quantity of car- 

 bonic acid in the atmosphere, which scarcely 

 amounts to 1-1 Oth per cent., sufficient for 

 the wants of the whole vegetation on the 

 surface of the earth, is it possible that the 

 carbon of plants has its origin from the air 

 alone? This question is very easily an- 

 swered. It is known, that a column of air 

 of 2441 Ibs! weight rests upon every square 

 Hessian foot (=0.567 square foot English) 

 of the surface of the earth ; the diameter of 

 the earth and its superficies are likewise 

 known, so that the weight of the atmosphere 

 can be calculated with the greatest exactness. 

 The thousandth part of this is caroonic acid, 

 which contains upwards of 27 per cent, car- 

 bon. By this calculation it can be shown, 

 that the atmosphere contains 3306 billion 

 Ibs. of carbon ; a quantity which amounts to 

 more than the weight of all the plants, and 

 of all the strata of mineral and brown coal, 

 which exist upon tne earth. This carbon 

 is, therefore, more than adequate to all the 

 purposes for which it is required. The 

 quantity of carbon contained in seawater is 

 proportionally still greater. 



If, for the sake of argument, we suppose 

 the superficies of the leaves and other green, 

 parts of plants, by which the absorption of 

 carbonic acid is effected, to be double that of 

 the soil upon which they grow, a supposi 



