DECAY OF WOODY FIBRE. 357 



ted yeast ; but a complete separation of the azotized 

 substances dissolved in it cannot take place when 

 air is excluded.* 



Neither alcohol alone, nor hops, nor indeed both 

 together, preserve beer from becoming acid. The 

 better kinds of ale and porter in England are pro- 

 tected 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 allowed to lie 

 for several years, so that they are treated in a man- 

 ner exactly similar to wine during its ripening. 



A gentle diffusion of air takes place through the 

 pores of the wood, but the quantity of azotized sub- 

 stances 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 W^OODY FIBRE. 



The conversion of w^oody fibre into the substances 

 termed humus and mould is, on account of its in- 

 fluence on vegetation, one of the most remarkable 

 processes of decomposition which occur in nature. 



Decay is not less important in another point of 



* The great influence which a rational management of fermentation 

 exercises upon the quahty of beer is well known in several of the Ger- 

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

 premium is offered for the preparation 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 becoming 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. — L. 



