CHher methods of waste disposal rest upon the prin-
ciple that " purification is an oxidizing process carried
out by bacteria living in the filter." The sprinkling or
trickling filter is one application of this idea. It is a
large bed of crushed stone upon which are thrown jets
of sewage which carries with it enough air to oxidize
the wastes. An accepted authority on this subject holds
that the trickling bed " appears to be the ideal method
of solving the essential problem of sewage disposal, the
oxidation of organic matter. It exhibits the simplicity
of all scientific applications which are merely intelligent
intensifications of natural processes. A pile of stones on
which the bacterial growth may gather, and a regulated
supply of sewage are the only desiderata. We meet the
conditions resulting from an abnormal aggregation of
human life in the city by setting up a second city of
microbes. The dangerous organic waste-material pro-
duced in the city of human habitations is carried out
to the city of microbes on their hills of rock, and we



Guarding the Health of the People 271

rely on them to turn it over into a harmless mineral
form."

This process of filtration is sometimes supplemented
by drainage through sand beds or chemical treatment.
