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organised worms, larvae, &c., which, like a collection of animals in
a zoological gardens, feed upon the daily supply of food given to
them, and so long as this supply is steadily and regularly maintained,
so long will they perform their life functions, and in so doing destroy
the waste organic matters which, in their collective form, we call
sludge; the process involved being merely that of digestion—the
waste products of one group being but food for a lower.
The process may very conveniently be watched from day to
day by placing the previously mentioned small piece of slate in an
ordinary plate or saucer, and putting on the "living earth" on the
slate small fragments of meat, bread, &c., and flooding the plate
with water gently so as to completely cover the whole. After an
hour or two gently decant the water from the plate so as to
thoroughly drain the plate and deposit thereon. Leave it freely
exposed to the air, preferably in a moderately warm atmosphere.
On watching from time to time it will be seen that the piece of red
meat has become coated with a grey deposit, this being often
complete in four or five hours. Now touch the grey matter with
the point of a wire, and transfer the minute quantity thus taken up
to a microscope slide as before, and examine under a high power,
when it will be seen that this grey matter is nothing but an
enormous number of bacteria, many in a state of restless activity
where they are not crowded in zooglea masses which prevent their
rapid movements.
Continue these observations from dav to day, each day flooding
with water and draining after two hours or so. In a few days the
solid particles of meat, &c., will become invisible and merged in
the mass of black humus into which it is finally resolved.
It is evident that this is precisely what takes place in a slate
bed. When the bed is first filled with sewage, and allowed to
stand full in a quiescent state for a couple of hours, the solid
matters settle on the slates. Until the "living earth" is fully
developed, the destructive action is slow, but in warm weather
especially the organisms rapidly develop and attack the food thus
provided for them, exactly as the organisms in a river mud attack
the matter deposited thereon from a tidal water receiving sewage
matters, and if the ratio of organisms, food and air supply are
properly regulated, the action proceeds indefinitely without the
evolution of nauseous odours.
The account given in the Tables of a series of experiments
made to ascertain the rate of destruction of various food stuffs when
placed on the mud on the slates will be of interest.