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37
in Poplar and Bromley was co-existent with the disease in the
Metropolis.

From these figures one must admit that diarrhœa has been appropriately
termed "the infant's destroying angel."
The high or low temperature of the air and of the soil, and the
amount of rainfall, have come to be considered as the conditions
causing diarrhœa. That one of the causes of the disease is due to
the processes of germ life going on in the upper layers of the soil
appears to be a fact proved from research, and that warmth is a
necessary factor, in order that the germs in the soil may propagate,
cannot be doubted. The disease will prevail in those places where
the soil is polluted with organic matter, e.g., houses built upon ground
made with all kinds of refuse, and built without having the site covered
with concrete, or without any underfloor ventilation, and the ground
where defective sewers and drains exist will form a suitable bed for
the germs to grow. Dr. Ballard has shewn that when the earth
four feet from the surface reaches a temperature of 56° F., and so
long as this temperature is maintained the epidemic prevalence of
diarrhœa continues, and commonly reaches its highest point about
the same time as the four feet earth temperature, but he points out
exceptions to this, we may say, rule, which need not be entered into
here.
Rainfall will affect the temperature of the soil, as the earth will
reach a higher temperature in dry weather than in wet. From Chart I.