66 THE CIECULATION OF NITEOGEN IN NATURE. 
material would be locked up, and be denied to us. Indeed^ 
the world, deprived of these useful Liliputians, would by this, 
time be peopled by the dead rather than by the living. 
But in the execution of this benignly destructive work it 
is of the greatest importance to us to note that the nitrogen 
is very largely converted into ammonia. Thus the following 
table shows the relative proportions of nitrogen as ammonia 
and as organic matter contained in water in which vegetable 
and animal tissues respectively were decaying : 
Vegetable. Animal. 
Ammoniacal nitrogen . . . -0085 3-387 ) Parts per 
Organic „ ... 1-287 -200) 100,000. 
Probably a great number of organisms are concerned in 
this process, but I will show photographs of two or three^ 
which are almost ubiquitous, as well as cultures on gelatine 
medium. • 
We now have our nitrogen in the shape of ammonia, a 
substance which in itself is useless either to animals or 
plants, but which, diffused in the superficial soil, falls- 
beneath the influence of two species of bacillus which 
present features of exceptional interest. These are the 
nitrifying bacteria. It had been known for centuries that 
when animal refuse mixed with an alkali was allowed to 
remain for a length of time with free access of air at a 
moderate temperature, a nitrate of the alkali was formed. 
In this way indeed saltpetre was always manufactured up 
to the time of the discovery of the natural deposits in Chili. 
It was supposed that the oxidation of the nitrogen of the 
animal matter was a purely chemical process. It was, I 
think, Pasteur who first suggested that a true fermentation 
was involved ; but in 1877 two French chemists proved this 
to be the case in the following manner: A gla^s tube was 
filled with a mixture of calcined sand and fragments of 
