PUTREFACTION. 617 



decay constantly occur. Where fluids putrefy for a long 

 time in cesspools, canals, gutters, &c. fermentation 

 and putrefaction ordinarily occur at first as the result 

 of the action of aerobic hacteria ; as the oxygen is 

 gradually used up, and as carbonic acid, hydrogen, and 

 other gases fill the fluid, a condition arises so favourable 

 for anaerobic bacteria as can scarcely ever be procured 

 by artificial means in pure cultivations. The anaerobes 

 then continue the putrefactive process, and more espe- 

 cially occasion the formation of large quantities of 

 stinking gases ; and, in addition to these putrefactive 

 anaerobes, other bacteria, which stand in no sort of 

 relation to the putrefactive process itself, such as the 

 bacilli of malignant oedema and of tetanus, utilise the 

 favourable conditions of complete exclusion of oxygen, 

 multiply, and form spores. 



A true decay, that is to say a putrefaction without Decay in the 

 the development of odorous gases and of reduction pro- sc 

 ducts, implies an extremely intimate contact of the 

 putrescible material with the air. The most favourable 

 conditions for this process seem to exist in soil which is 

 very porous, and is moistened from time to time ; under 

 such circumstances mineralisation of organic substances 

 occurs in such a complete manner that in a short time 

 neither ammonia nor sulphuretted hydrogen, nor more 

 complex carbon compounds are present, but on the con- 

 trary only nitrates, sulphates, and carbonic acid. For 

 further remarks as to nitrification see the chapter on 

 "Soil." We very frequently meet with a mixture of 

 putrefaction and decay in dead organic materials. In 

 the upper layers of a fluid, or on the surface of a dead 

 body, decay can occur, while in the deeper layers putre- 

 factive processes of such extent and intensity take place 

 that the products of true putrefaction appear side by 

 side with the products of decay. 



In the decomposition in the soil we frequently observe Mouldiness 

 products which can not as yet be satisfactorily defined; (vcr 

 these are more especially seen in the destruction of portions 

 of plants which chiefly consist of cellulose. In this case 

 there is a formation of humin substances, and when oxygen 



