Medical Officers of the Army of India.

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ditions; they set up the putrefactive fermentation proper, but they require the pre-
paratory action of the aërobes. If, however, the fluid is exposed to the air, the
aërobes develop continuously at the surface and form a scum which at times
falls to the bottom in masses, but is constantly regenerated. The scum prevents
the access of oxygen to the liquid, and thus provides the necessary anaërobic
conditions for the putrefactive organisms below. The complex fermentative pro-
ducts, thus formed, serve as nutriment to the aërobes at the surface and the
latter break them up into the simplest compounds, water, carbon dioxide and
ammonia. [Sims Woodhead has since confirmed this, by the demonstration of
anaërobes in the lower part and body of an open tank, and of liquefying aerobes
on the surface, to the rôle of which in the second stage we have referred.] In
these words we have a prophetic statement of the advantages to be obtained
from the treatment of sewage in an open "Septic tank," or in an open upward
"cultivation tank" (Scott-Moncrieff) to which we shall subsequently refer.
But it is obvious that, in practice, in dealing with large bodies of sewage, we
cannot rely on this process alone for the sufficiently rapid mineralization of the
waste products; we should not only need apparatus of a size impossible to obtain,
but we should incur all the disadvantages and dangers of the old cess-pool
system, with loss of the immense manurial value of the organic matter.
Nevertheless it is clear that in fermentation without oxygen we have so far gained
enormous advantages in the breaking down and solution of the solid waste, which
is now rendered far more readily susceptible of final oxidation.

   Coming now to the practical application of the principles which govern
the biochemical forces in action, we see that we must provide the following
stages for their distinct rôles:—

The practical
application.

      (1) For anaërobic liquefaction and solution of the solids—albuminous,
carbohydrate and fatty. This is best accomplished in what is
called a "septic tank," closed or open, or by upward filtration
on the Scott-Moncrieff plan, which resembles in all essentials the
open septic tank, but possesses certain important advantages in
that it provides a far better nidus for the organisms and an enor-
mously increased area for their cultivation under suitable conditions;
it also allows of the initiation of the second stage of partial oxida-
tion, and by so much saves time, and the stress on the apparatus
provided for the second stage. (See Part III.)

      (2) The second stage in which the intermediate products are broken
down by fermentation in the presence of a certain amount of
oxygen, and here intermittent downward "filtration" is indicated.