CH. XIl] THE CIRCULATION OF NITROGEN 277 



earliest Egj^ptian mummies were prepared, frozen in the soil of 

 Northern Siberia, and with the flesh so " fresh " that it has been 

 eaten by do^s. But these are quite exceptional conditions, and it 

 may be stated quite generally that as soon as a plant or animal 

 dies, the organic substance of its body is broken down by the action 

 of putrefactive bacteria. 



The precise nature and course of putrefactive decomposition 

 under different conditions are not at all thoroughly investigated. 

 The decomposition of fats and carbohydrates is however a com- 

 paratively simple process. The former substances are decomposed 

 into the glycerine and fatty acids of which they are made up, and 

 then each of these products is further broken down into simpler 

 organic acids, carbon dioxide and water. Ultimately these bacterial 

 decompositions or fermentations proceed so far that the whole fat 

 or carbohydrate is resolved into the two compounds, water and 

 carbon dioxide, with perhaps traces of other gases, such as methane. 

 In the case of the proteids however, the process of putrefaction 

 is much more complicated, and a multitude of highly complex 

 chemical substances are elaborated. Among these are albumoses 

 and peptones, then ptomaines and amido-acids, evil-smelling 

 substances such as indol, skatol and phenol, and simpler compounds 

 like sulphuretted hydrogen, methane, ammonia, volatile amines, 

 carbon dioxide, &c. The precise order of appearance, and pro- 

 portions of these products, vary with the external conditions of the 

 putrefaction. Most of the substances mentioned are however 

 unstable, and are soon resolved into much simpler compounds. 

 There is little or no difference between the putrefaction products 

 of the nitrogenous constituents of the bodies of animals and 

 plants, and probably no difference at all in the nature of the 

 decomposition of their fats and carbohydrates. Even the relatively 

 resistant cellulose of the plant ultimately is broken down by 

 fermentation bacteria as in the case of the carbohydrate of the 

 animal body. 



It is usually assumed that the changes which take place in 

 organic matter undergoing putrefaction are due entirely to the 

 activity of bacteria. There is, of course, little doubt that the 

 characteristic organisms of a putrefying mass belong to this group 

 of micro-organisms. If we allow a small piece of flesh to stand 



