Medical Officers of the Army of India.

131

     By the recognition of these laws the modern technical applications of
bacteriology have secured their greatest triumphs, as in brewing, wine ferment-
ation, tanning, the making of butter and cheese, &c., and they have a most important
application to scientific methods of sewage "purification." Summing up at this
stage, we see, then, that bacteria are able to construct their body-substance out
of various kinds of nutrient materials, analytically or synthetically, and at the
same time to act with marked effect upon the substratum. But vital differences
in these respects are to be noted; some species are better adapted to one method
and one set of conditions, which arc foreign to others. In the decomposition of
dead organic matter certain species are better fitted to break down the complex
compounds present at the start, while others work up the end products to final
oxidation, the complete drama being presented in a series of integral acts, in
which each species plays a defined rôle.

     A brief allusion may now be made to the special nature of the vital and
chemical actions of bacteria, in virtue of which the whole of the enormous and
never-ceasing accumulation of waste organic products is transformed and enabled
to re-enter the cycle of life.

The biochemical
forces in the
transformation
of waste pro-
ducts.

     For our present purpose, these waste products may be distinguished as
nitrogenous (urea, waste proteids and their derivatives), and non-nitrogenous
(chiefly carbohydrates). As regards the former we know that the nitrogen con-
tained in the urea and in the solid excrement is not in a form available for
plant food. It is not until this nitrogen has been removed from the organic molecule
by the process of putrefaction, and has been united to a mineral base by the
subsequent process of nitrification, that it can be disposed of by the plant. Putre-
faction is a purely biochemical process, which only takes place under the
fundamental conditions of all vital action—temperature, moisture, etc., for it is the
work of bacteria and of bacteria only, which possess the power of breaking
up the complex proteid molecule into simpler groups. But we must recognize
that in practice (i .e ., in dealing with sewage) we never have a purely proteid
matter to work upon; there are also non-nitrogenous matters (cellulose, etc.),
which are resolved by a series of definite fermentations; these actions go on
side by side, and although nitrogenous bodies are putrescible and non-nitrogen-
ous bodies are non-putrescible, we cannot distinguish a fundamental difference in
the vital forces at work in true putrefaction and in fermentation respectively, and
as a supply of nitrogen is absolutely necessary to the agents of the fermentation,
the processes are, in practice, indivisible. Both depend on biochemical forces
inherent in the living cells, and further to some extent on their secretions
which are called "enzymes," which bring about changes in the substratum

Putrefaction.

Fermentation and
enzymes.