the walls of a crowded room is potentially a productive focus for
microbes. Every deposit of dirt on persons, clothing, or furniture is
also a productive focus, and production is fostered in close apartments
by the warmth and moisture of the place. In hospitals productive foci are
more numerous than in ordinary dwellings.

If microbes are present in the breath of ordinary individuals, what can
we expect in the breath of those whose lungs are rotten with tubercular
disease? Then we have the collections of expectorated matter and of other
organic secretions, which all serve as productive foci. Every wound and
sore, when antiseptic precautions are not used, becomes a most active and
dangerous focus, and every patient suffering from an infective disease is
probably a focus for the production of infective particles. When we
consider, also, that hospital wards are occupied day and night, and
continuously for weeks, it is not to be wondered at that microbes are
abundant therein.

I want especially to dwell upon the fact that foci, and probably
productive foci, may exist outside the body. It is highly probable,
judging from the results of experiments, that every collection of
putrescible matter is potentially a productive focus of microbes. The
thought, of a pit or sewer filled with excremental matters mixed with
water, seething and bubbling in its dark warm atmosphere, and
communicating directly (with or without the intervention of that
treacherous machine called a trap) with a house, is enough to make one
shudder, and the long bills of mortality already chargeable to this
arrangement tell us that if we shudder we do not do so without cause. As
an instance of the way in which dangers may work in unsuspected ways, I
may mention the fact that Emmerich, in examining the soil beneath a ward
of a hospital at Amberg, discovered therein the peculiar bacillus which
causes pneumonia, and which had probably been the cause of an outbreak of
pneumonia that had occurred in that very ward.

The importance of "Dutch cleanliness" in our houses, and the abolition of
all collections of putrescible matter in and around our houses, is
abundantly evident.

It will not be without profit to examine some well-known facts, by the
aids of the additional light which has been thrown upon them by the study
of the microbes which are in the media around us.

There is no better known cause of a high death rate than overcrowding.
Overcrowding increases t