68 THE CIRCULATION OF NITROGEN IN NATURE. 
general^ lime or magnesia, with which the nitrous and 
nitric acids can combine. At the same time, the alkalinity 
must be feeble, a bicarbonate of an alkali or alkaline earth 
being the best form of base. They differ from most known 
species, not only in their complete independence of organic 
food, but also in their peculiar invisibility, water in which 
they are quite abundant being perfectly bright, and even 
brilliant. 
They differ from one another in several important particu- 
lars. The nitrous organism is a rather stout, oval bacillus, 
developing rapidly in water containing an ammonium salt, 
such as the phosphate of soda and ammonia. 
The nitric organism is a narrow, small bacillus, often peg- 
top shaped ; it is very intolerant of ammonia, and does not, 
therefore, come into play if much ammonia is present, but 
remains latent until most of the ammonia has been con- 
verted into nitrite. In ordinary soil, however, the conditions 
are generally such as to allow of both fermentations pro- 
ceeding, so that the nitrite is oxidized almost as soon as 
formed. " : ^ vr;;;'- - 
I exhibit here the older form of experiment showing the 
fermentations proceeding in flasks, also a filter representing 
a section of soil, and similar to that just referred to. 
In this way our nitrogen is rendered available for plant 
food generally, for it is as nitrate that nitrogen is taken up 
by plants from the soil. The necessary bacteria are univer- 
sally present in the superficial soil, abundantly to a depth of 
about eighteen inches, more sparingly to a depth of about 
eight feet. In the light of these researches it is possible 
to understand what guides the farmer in his choice of a 
nitrogenous manure. If he. wishes to get an immediate 
effect he chooses a nitrate ; if he requires the action to extend 
over a slightly longer period he applies au ammoniacal 
