vii.] CELLULOSE FERMENTATION 129 



time. Carbon dioxide will always be found, but if that 

 is removed by shaking up the contents of the tube with 

 lime water or a little caustic soda, some other gas will 

 also remain, and this geis will take fire if a light be 

 brought to the mouth of the test-tube. The inflammable 

 gas is generally marsh-gas, sometimes hydrogen, or a 

 mixture of the two. At the same time, it will be seen 

 that the filter paper is being attacked; the strips get 

 thinner and begin to break up in places ; finally they 

 disappear entirely, or give place to a dark brown, 

 structureless mass. On testing the liquid it will be 

 found to be acid. In this type of fermentation, which 

 takes place without any contact with the air, cellulose 

 and other carbohydrates become split up into carbon 

 dioxide and either marsh-gas or hydrogen, a certain 

 amount of humus being formed at the same time. It 

 should be noted that all the changes described in this 

 and the preceding experiments will not take place if the 

 action of bacteria is suspended at starting by intro- 

 ducing chloroform into the mixture, or by sterilising the 

 whole by boiling ; they are all processes due to living 

 agencies, and in most cases we cannot reproduce them 

 in the laboratory by purely inanimate means. These 

 two modes of breaking down carbohydrates illustrate 

 processes which are widespread in nature. The first, 

 in which the carbon compounds are oxidised in the 

 presence of air to carbon dioxide, represents the slow 

 decay which overtakes all organic matter when freely 

 exposed. A leaf, a branch, even a tree itself, which falls 

 to the ground in a wood is eventually dissipated into 

 gas, and leaves behind nothing more solid than the ash 

 which would have resulted if it had been burnt. The 

 process of aerial decay is, in fact, neither more nor less 

 than a slow burning brought about by living organisms ; 

 not only bacteria effect such actions, but many fungi, 



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