BACTERIA AND PROTOZOA 27 



taking place ; but whereas in the bonfire a tempera- 

 ture is reached at which the oxygen of the air is able 

 directly to combine with the Carbon compounds and 

 break them down into Carbon dioxide and Water, in 

 the manure-heap a similar but slower and less com- 

 plete process is occurring. This change is the 

 earliest to be noticed, and though other changes are 

 happening at the same time, they are more logically 

 considered as being later. In the first instance the 

 bacteria there are varieties that can do the work 

 in the presence of air if the heap is loosely com- 

 pacted, and varieties that can accomplish it if all 

 air is excluded break down the complex Carbon 

 compounds, turning them into simpler bodies, and 

 partly into Carbon dioxide that escapes into the air. 

 These nations of bacteria wax and wane, giving place 

 to three separate groups that one after another deal 

 with the nitrogen compounds. The bacteria called 

 upon to deal with these bodies have to be fed with 

 readily assimilable carbon compounds. The first 

 group, some of whose members can work with air 

 and others without, seize on the nitrogen fixed in 

 complex groupings and split off ammonia from the 

 protein (NH 3 ). Their finished product they hand 

 on to another group. These can only work in the 

 presence of air, and they require lime or some basic 

 material to absorb the acid they produce. Under 

 their influence the ammonia they have had passed 

 to them as the raw material (often in the form of 

 Ammonium Sulphate [(NH 4 ) 2 SO 4 ]) is changed into 

 nitrite (NO 2 ), and this by a third group, also acting 

 in the air, and in the presence of a base becomes 



