DECOMPOSITION OF ORGANIC COMPOUNDS. 293 



ments may be divided into two classes : 1. Soluble ferments, which 

 are in most cases nitrogenous substances, closely related to the pro- 

 teids ; 2. Living micro-organisms of either vegetable or animal origin. 



The nature of the ferment generally 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 decom- 

 pose 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 con- 

 taining, in addition to these three elements, either nitrogen or sul- 

 phur (or both) undergo putrefaction. The two last-named elements 

 are generally evolved as ammonia or derivatives of ammonia and 

 hydrogen sulphide, which gases give rise to an offensive odor. 



Sugar, having the composition C 6 H 12 O 6 , undergoes fermentation, 

 whilst albuminous substances which contain also nitrogen and sul- 

 phur 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 favorable 

 conditions. 



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

 composition in nature, the processes of fermentation and putrefaction 

 are generally accompanied by oxidation or decay. 



The conditions under which a substance will ferment or putrefy 

 have been stated above, and the non-fulfilment of these conditions 

 enables us to prevent decomposition artificially. 



Thus, we freeze substances (meat) ; or expel all water from or dry 

 them (fruit, etc.), in order to prevent decomposition. The action of 

 the ferments is counteracted either by the so-called antiseptic agents 

 (salt, carbolic or salicylic acid, etc.) which are incompatible with 

 organic life, or by excluding the air, and with it the ferments, by 

 enclosing the substances in air-tight vessels (glass jars, tin cans, etc.), 

 which, when filled, are heated sufficiently to destroy any germs which 

 may have been present. 



