THE CIKCULATION OF NITROGEN IN NATURE. 69 
manure which has to pass through the stage of nitrification 
before it becomes available, and in this he is assisted by a 
curious difference in the behaviour of soils towards the two 
classes, viz., that whereas nitrates are very rapidly washed 
away, and may be lost if carried beyond the range of action 
of the roots, ammonia on the contrary is retained by the 
soil, and not removed bodily by water, but only gradually 
becomes available under the influence of the nitrifying 
ferments. If nitrogenous organic substances are applied, 
of course a still longer preparation in the shape of putre- 
faction is required before the contained nitrogen can become 
assimilable. 
But the microbic action does not end here. Nitrates, if 
Tinabsorbed by plants, sink with the drainage water into the 
subsoil and rapidly disappear. For if we bore into culti- 
vated and manured land, and take samples of the water 
percolating into the boring at different depths, we shall find 
that the nitrates so abundant at and near the surface 
speedily diminish, until at a depth of upwards of 100 feet 
they have practically disappeared. - - - ■ 
Now this change is chiefly, if not entirely, brought about 
by organisms similar in many respects to those effecting 
nitrification, but which have not an oxidizing, but a reducing 
power. This was supposed at one time to be due to a single 
species, B. denitrijicans ; but the reducing power has been 
shown to be common to a great number of species, an 
interesting instance being afforded by the cholera spiril- 
lum. 
In this way an inverse series of changes results, and 
finally the nitrogen is liberated as the free element in the 
gaseous condition, ultimately reaching the atmosphere dis- 
solved in the water of deep springs, the gas evolved from 
which, as in the case of our Hotwell, consists very largely 
