FERMENTATION. 137 



present. Here, as in the case of the yeasts, we find both 

 aerobic and anaerobic organisms. The aerobic forms give 

 rise to oxidation of certain of the products of decomposi- 

 tion, one of these, nascent hydrogen, especially taking up 

 oxygen, as a result of which water is formed ; the an- 

 aerobic organisms, which may be said to complete the 

 process of putrefactive fermentation, only commence to grow 

 when the aerobic organisms have used up all the readily 

 available oxygen such as that dissolved in a liquid, on the 

 surface of which there has formed a kind of film, by the 

 intervention of which aeration from the external air is greatly 

 interfered with. 



We have already noted this film growing on yeasts, when it 

 appears to assist the carbon dioxide developed during the pro- 

 cess of fermentation in keeping off the oxygen of the atmo- 

 sphere, and to allow of these anaerobic organisms carrying on 

 their growth without the further intervention of a free oxidiz- 

 ing agent. The products of these anaerobic organisms, of 

 course, differ from the products of the aerobic fermentations 

 in so far that they contain less oxygen, the sulphuretted 

 hydrogen, hydrogen, and nitrogen being released in a more 

 or less free condition and uncombined with oxygen, the 

 oxygen of the original fluid being used up in the formation 

 of such substances as leucine, CsH,o(NHj)CO OH ; tyrosine 



QhJ^ (NHJCO, OH) ; the volatile fatty acids ; some 



compound ammonias, and, of course, carbon dioxide ; the 

 two latter of which, as already seen, may be formed from 

 urea (CO(HN')s) by simple hydration, the more highly 

 organized substances, such as leucine and tyrosine, being, 

 during the process of fermentation, further converted into 

 ammonia and the fatty acids. 



From the variety of the products of these fermentations 

 it is evident that we have to do, not with a simple conver- 

 sion by means of any single organism into the simplest and 

 ultimate elements of decomposition, but that we have a 

 series of breakings down or stages of decomposition by which 

 the highest (and most unstable) organic materials are 

 gradually transformed into the lowest and most stable. It 

 will be. found, however, that the process is not quite so 

 straightforward and simple as above stated, as some of the 

 energy released during the formation of a number of lower 



