bacteria can build up organic matter from purely mineral
sources by assimilating carbon from carbon dioxide in the dark and by
obtaining their nitrogen from ammonia. The energy liberated during the
oxidation of the nitrogen is regarded as splitting the carbon dioxide
molecule,--in green plants it is the energy of the solar rays which does
this. Since the supply of free oxygen is dependent on the activity of green
plants the process is indirectly dependent on energy derived from the sun,
but it is none the less an astounding one and outside the limits of our
previous generalizations. It has been suggested that urea is formed by
polymerization of ammonium carbonate, and formic aldehyde is synthesized
from CO_2 and OH_2. The _Nitro-bacteria_ are smaller, finer and quite
different from the nitroso-bacteria, and are incapable of attacking and
utilizing ammonium carbonate. When the latter have oxidized ammonia to
nitrite, however, the former step in and oxidize it still further to nitric
acid. It is probable that important consequences of these actions result
from the presence of nitrifying bacteria in rotten stone, decaying bricks,
&c., where all the conditions are realized for preparing primitive soil,
the breaking up of the mineral constituents being a secondary matter. That
"soil" is thus prepared on barren rocks and mountain peaks may be concluded
with some certainty.

[Illustration: FIG. 14.--Stages in the formation of a colony of a variety
of _Bacillus (Proteus) vulgaris_ (Hauser), observed in a hanging drop. At
11 A.M. a rodlet appeared (A); at 4 P.M. it had grown and divided and
broken up into eight rodlets (B); C shows further development at 8 P.M., D
at 9.30 P.M.--all under a high power. At E, F, and G further stages are
drawn, as seen under much lower power. (H. M. W.)]

In addition to the bacterial actions which result in the oxidization of
ammonia to nitrous acid, and of the latter to nitric acid, the reversal of
such processes is also brought about by numerous bacteria in the soil,
rivers, &c. Warington showed some time ago that many species are able to
reduce nitrates to nitrites, and such reduction is now known to occur very
widely in nature. The researches of Gayon and Dupetit, Giltay and Aberson
and others have shown, moreover, that bacteria exist which carry such
reduction still further, so that ammonia or even free nitrogen may escape.
The importance of these results is evident in explaining an old puzzle