254 CONSIDERATION OF CARBON COMPOUNDS. 



temperatures. No substance, however, will either ferment or 

 putrefy at or below the freezing-point, or at or above the boiling- 

 point. 



The nature of the various ferments differs widely, and their 

 true action cannot, in many cases, be explained ; what we do 

 know is, that the presence of comparatively small (often minute) 

 quantities of one substance (the ferment) is sufficient to cause 

 the decomposition of large quantities of certain organic sub- 

 stances, the ferment itself suffering often no apparent change 

 during this decomposition. The ferments are, in a few cases, 

 organic substances, but generally living organisms of either 

 vegetable or animal origin. 



The nature of the ferment frequently determines the nature 

 of the decomposition which a substance suffers, or, in other 

 words, one and the same substance will under the influence of 

 one ferment decompose with liberation of certain products, 

 while a second ferment causes other products to be evolved. 

 Sugar, for instance, under the influence of yeast, is converted 

 into alcohol and carbon dioxide, while under the influence of 

 certain other ferments it is converted into lactic acid. 



The difference between fermentation and putrefaction is, that 

 the first term is used in those cases where the decomposing 

 substance contains carbon, hydrogen, and oxygen only, while 

 substances containing in addition to these three elements either 

 nitrogen or sulphur (or both) undergo putrefaction. The two 

 last-named elements are generally evolved as ammonia and 

 hydrosulphuric acid, which gases give rise to an offensive odor. 



Sugar, having the composition 6 H 12 O 6 , undergoes fermenta- 

 tion, whilst albuminous substances which contain nitrogen and 

 sulphur putrefy. 



The oxygen of the air takes no part in either fermentation or 

 putrefaction, but the presence or absence of atmospheric air 

 may cause or prevent decomposition, inasmuch as the atmosphere 

 is filled with millions of minute germs of organic nature, which 

 germs may act as ferments when in contact with organic matter 

 under otherwise favorable conditions. 



Whenever organic bodies (a dead animal, for instance) undergo 

 decomposition in nature, the processes of fermentation and 

 putrefaction are generally accompanied by oxidation or decay. 



The conditions under which a substance will ferment or 



