d others were right, and that _Clostridium
pasteurianum_, for instance, if protected from access of free oxygen by an
envelope of aerobic bacteria or fungi, and provided with the carbohydrates
and minerals necessary for its growth, fixes nitrogen in proportion to the
amount of sugar consumed. This interesting case of symbiosis is equalled by
yet another case. The work of numerous observers has shown that the free
nitrogen of the atmosphere is brought into combination in the soil in the
nodules filled with bacteria on the roots of Leguminosae, and since these
nodules are the morphological expression of a symbiosis between the higher
plant and the bacteria, there is evidently here a case similar to the last.

As regards the ammonium carbonate accumulating in the soil from the
conversion of urea and other sources, we know from Winogradsky's researches
that it undergoes oxidation in two stages owing to the activity of the
so-called "nitrifying" bacteria (an unfortunate term inasmuch as
"nitrification" refers merely to a particular phase of the cycle of changes
undergone by nitrogen). It had long been known that under certain
conditions large quantities of nitrate (saltpetre) are formed on exposed
heaps of manure, &c., and it was supposed that direct oxidation of the
ammonia, facilitated by the presence of porous bodies, brought this to
pass. But research showed that this process of nitrification is dependent
on temperature, aeration and moisture, as is life, and that while
nitre-beds can infect one another, the process is stopped by sterilization.
R. Warington, J. T. Schloessing, C. A. Muentz and others had proved that
nitrification was promoted by some organism, when Winogradsky hit on the
happy idea of isolating the organism by using gelatinous silica, and so
avoiding the difficulties which Warington had shown to exist with the
organism in presence of organic nitrogen, owing to its refusal to nitrify
on gelatine or other nitrogenous media. Winogradsky's investigations
resulted in the discovery that two kinds of bacteria are concerned in
nitrification; one of these, which he terms the _Nitroso-bacteria_, is only
capable of bringing about the oxidation of the ammonia to nitrous acid, and
the astonishing result was obtained that [v.03 p.0165] this can be done, in
the dark, by bacteria to which only pure mineral salts--_e.g._ carbonates,
sulphates and chlorides of ammonium, sodium and magnesium--were added. In
other words these