no 



AGRICULTURAL CHEMISTRY. 



the fluiu diminish when the greater part of 

 the sugar has undergone decomposition; 

 but they must cease entirely before the pro- 

 rets can be regarded as completed. 



The less rapid process of decomposition 

 which succeeds the violent evolution of gas, 

 continues in wine and beer until the sugar 

 has completely disappeared ; and hence it is 

 observed, that the specific gravity of the 

 liquid diminishes during many months. 

 This slow fermentation, in most cases, re- 

 sembles the fermentation of Bavarian beer, 

 the transformation of the dissolved sugar 

 being in part the result of a slow and con- 

 tinued decomposition of the precipitated 

 yeast; but a complete separation of the 

 azotised substances dissolved in it cannot 

 take place when air is excluded.* 



Neither alcohol alone, nor hops, nor in- 

 deed both together, preserve beer from be- 

 coming acid. The better kinds of ale and 

 porter in England are protected from acidity, 

 but at the loss of the interest of an immense 

 capital. They are placed in large closed 

 wooden vessels, the surfaces of which are 

 covered with sand. In these they are al- 

 lowed to lie for several years, so that they 

 are treated in a manner exactly similar to 

 wine during its ripening. 



A gentle diffusion of air takes place 

 through the pores of the wood, but the quan- 

 tity of azotised substances being very great 

 in proportion to the oxygen which enters, 

 they consume it, and prevent its union with 

 the alcohol. But the beer treated in this 

 way does not keep for two months without 

 acidifying, if it be placed in smaller vessels, 

 to which free access of the air is permitted. 



CHAPTER X. 



DECAY OF WOODY FIBRE. 



THE conversion of woody fibre into the 

 substances termed humus and mould is, on 

 account of its influence on vegetation, one 

 of the most remarkable processes of decom- 

 position which occur in nature. 



Decay is not less important in another 

 point of view ; for, by means of its influ- 

 ence on dead vegetable matter, the oxygen 

 fchich plants retained during life is again 

 restored to the atmosphere. 



The decomposition of woody fibre is ef- 

 fected in three forms, the results of which 



* The great influence which a rational manage- 

 ment of fermentation exercises upon the quality 

 of beer is well known in several of the German 

 states. In the grand-duchy of Hesse, for example, 

 a considerable premium is offered for the prepa- 

 ration of beer, according to the Bavarian method ; 

 and the premium is to be adjudged to any one 

 who can prove that the beer brewed by him has 

 lain for six months in the store-vats without be- 

 coming acid. Hundreds of casks of beer became 

 changed to vinegar before an empirical knowledge 

 of those conditions was obtained, the influence of 

 which is rendered intelligible by the theory. 



are different, so that it is necessary to con- 

 sider each separately. 



The first lakes place when it is in the 

 moist condition, and subject to free uninter- 

 rupted access of air; the second occurs 

 when the air is excluded; and the third 

 when the wood is covered with water, and 

 in contact with putrefying organic matter. 



It is known that woody fibre may be kept 

 under water, or in dry air, for thousands of 

 years without suffering any appreciable 

 change; but that when brought into contact 

 with air, in the moist condition, it converts the 

 oxygen surrounding it into the same volume 

 of carbonic acid, and is itself gradually 

 changed into a yellowish brown, or black 

 matter, of a loose texture.* 



It has already been mentioned, that pure 

 woody fibre contains carbon and the ele- 

 ments of water. Humus, however, is not 

 produced by the decay of pure woody fibre, 

 but by that of wood which contains foreign 

 soluble and insoluble organic substances, 

 besides its essential constituents. 



The relative proportion of the component 

 elements are, on this account, different in 

 oak wood and in beech, and the composition 

 of both of these differs very much from 

 woody fibre, which is the same in all vege- 

 tables. The difference, however, is so tri- 

 vial, that it may be altogether neglected in 

 the consideration of the questions which 

 will now be brought under discussion ; be- 

 sides, the quantity of the foreign substances 

 is not constant, but varies according to the 

 season of the year. 



According to the careful analysis of Gay- 

 Lussac and Thenard, 100 parts of oak wood, 

 dried at 212 (100 C.,) from which all 

 soluble substances had been extracted by 

 means of water and alcohol, contained 

 52-53 parts of carbon, and 47-47 parts of 

 hydrogen and oxygen, in the same propor- 

 tion as they are contained in water. 



Now it has been mentioned that moist 

 wood acts in oxygen gas exactly as if its 

 carbon combined directly with oxygen, and 

 that the products of this action are carbonic 

 acid and humus. 



If the action of the oxygen were confined 

 to the carbon of the wood, and if nothing 

 but carbon were removed from it, the re- 

 maining elements would necessarily be 

 found in the humus, unchanged except in 

 the particular of being combined with less 

 carbon. The final result of the action would 

 therefore be a complete disappearance of the 

 carbon, whilst nothing but the elements of 

 water would remain. 



But when decaying wood is subjected to 

 examination in different stages of its decay, 



* According to the experiments of DC Saussure, 

 240 parts of dry sawdust of oak wood convert 10 

 cubic inches of oxygen into the same quantity of 

 carbonic acid, which contains 3 parts, by weight, 

 of carbon ; while the weight of the sawdust is di- 

 minished by 15 parts. Hence, 12 parts, by weight, 

 of water, are at the same time separated from the 

 elements of the wood. 



