AGRICULTURAL CHEMISTRY. 



Buchner found a brown substance soluble 

 in alkalies. This substance was evidently 

 due to the secretions from the roots of the 

 plants which grew in it. 



A plant placed in a closed vessel in which 

 the air, and therefore the carbonic acid, can- 

 not be renewed, dies exactly as it would do 

 in the vacuum of an air-pump, or in an at- 

 mosphere of nitrogen or carbonic acid, even 

 though its roots be fixed in the richest mould. 



Plants do not, however, attain maturity, 

 under ordinary circumstances, in charcoal 

 powder, when they are moistened with pure 

 distilled water instead of rain or river water. 

 Rain water must, therefore, contain within 

 it one of the essentials of vegetable life ; and 

 it will be shown, that this is the presence of 

 a compound containing nitrogen, the exclu- 

 sion of which entirely deprives humus and 

 charcoal of their influence upon vegetation. 



CHAPTER IV. 



ON THE ASSIMILATION OF HYDROGEN. 



THE atmosphere contains the principal 

 food of plants in the form of carbonic acid, 

 in the state, therefore, of an oxide. The 

 solid part of plants (woody fibre) contains 

 carbon and the constituents of water, or the 

 elements of carbonic acid, together with a 

 certain quantity of hydrogen. It has for- 

 merly been mentioned that water consists of 

 the two gases, oxygen and hydrogen. The 

 range of affinity possessed by both these 

 elements is so extensive that numerous 

 causes occur which effect the decomposition 

 of water. Indeed, there is no compound 

 which plays a more general or more im- 

 portant part in the phenomena of combina- 

 tion and decomposition. We can conceive 

 the wood to arise from a combination of the 

 carbon of the carbonic acid with the elements 

 of water, under the influence of solar light. 

 In this case, 72.35 parts of oxygen, by weight, 

 must be separated as a gas for every 27.65 

 parts of carbon, which are assimilated by a 

 plant; for this is the composition of carbonic 

 acid in 100 parts. Or, what is much more 

 probable, plants, under the same circum- 

 stances, may decompose water, the hydro- 

 gen of which is assimilated along with' car- 

 bonic acid, whilst its oxygen is separated. 

 If the latter change takes place, 8.04 parts 

 of hydrogen must unite with 100 parts of 

 carbonic acid, in order to form woody fibre, 

 and the 72.35 parts by weight of oxygen, 

 which was in combination with the hydro- 

 gen of the water, and which exactly corre- 

 sponds in quantity with the oxygen contained 

 in the carbonic acid, must be separated in a 

 gaseous form. 



Each acre of land, which produces 10 

 cwts. of carbon, gives annually to the at- 

 mosphere 865 Ibs. of free oxygen gas. The 

 specific weight of oxygen is expressed by 

 the number 1.1026; hence 1 cubic metre of 



oxygen weighs 3.157 Ibs., and 2865 Ibs. of 

 oxygen correspond to 908 cubic metres, or 

 32,007 cubic feet. 



An acre of meadow, wood, or cultivated 

 land in general replaces, therefore, in the 

 atmosphere as much oxygen as is exhausted 

 by 10 cwts. of carbon, either m its ordinary 

 combustion in the air or in the respiratory 

 process of animals. 



It has been mentioned at a former page 

 that pure woody fibre contains carbon and 

 the component parts of water, but that ordi- 

 nary wood contains more hydrogen than 

 corresponds to this proportion. This excess 

 is owing to the presence of the green princi- 

 ple of the leaf, wax., resin, and other bodies 

 rich in hydrogen. Water must be decom- 

 posed, in order to furnish the excess of this 

 element, and consequently one equivalent of 

 oxygen must be given back to the atmosphere 

 for every equivalent of hydrogen appropri- 

 ated by a plant to the production of those sub- 

 stances. The quantity of oxygen thus set at 

 liberty cannot be insignificant, for the at- 

 mosphere must receive 989 cubic feet of 

 oxygen for every pound of hydrogen assi- 

 milated. 



It has already been stated, that a plant, in 

 the formation of woody fibre, must always 

 yield to 'the atmosphere the same propor- 

 tional quantity of oxygen ; that the volume 

 of this gas set free would be the same 

 whether it were due to the decomposition of 

 carbonic acid or of water. A little consi- 

 deration will show that this must be the case. 

 It has repeatedly been stated, that woody 

 fibre contains carbon in combination with 

 oxygen and hydrogen in the same propor- 

 tion in which they exist in water. Water 

 contains 1 equivalent of each element, whilst 

 carbonic acid consists of 1 equivalent of 

 carbon, united to 2 equivalents of oxygen. 

 In the formation of woody fibre, 2 equiva- 

 lents of oxygen must therefore be libe- 

 rated. The woody fibre can only be 

 formed in one of two ways : either the car- 

 bon of carbonic acid unites directlv with 

 water, or the hydrogen of water combines 

 with the oxygen of the carbonic acid. In 

 the former of these cases, the two equiva- 

 lents of oxygen in the carbonic acid must be 

 liberated ; in the latter, two atoms of wate~ 

 must be decomposed, the hydrogen of which 

 unites with the oxygen of the carbonic acid, 

 whilst the oxygen of the water, thus set 

 free, is disengaged in the state of a gas. It 

 was considered most probable that the latter 

 was the case. 



From their generating caoutchouc, wax, 

 fats, and volatile oils containing nydrogen 

 in large quantity, and no oxygen, we may 

 be certain that plants possess the rjroperty 

 of decomposing water, because from no 

 other body could they-obtain the hydrogen 

 of those matters. It has also been proved 

 by the observations of Humboldt on the 

 fungi, that water may be decomposed with- 

 out the assimilation of hydrogen. Water 13 

 a remarkable combination of two elements. 



