in
water during cold weather, as experiments have shown that dissolved
oxygen favors longevity.

Still another reason for the larger numbers of bacteria that pass
through a water filter during cold weather may be the effect that
the low temperature has on the size of the bacteria themselves. A
few experiments made recently by the writer appear to indicate that
at low temperatures the gelatinous membrane which surrounds the
bacterial cells tends to become somewhat contracted, thus decreasing
the apparent size of the bacteria as seen under the microscope.
Either this contraction occurs, or the cells themselves are smaller
when they develop in the cold. It is possible also that low
temperature affects the flagella of the organisms in the same way.
It is not unreasonable to suppose that the effect of low temperature
is to form what may be, in effect, a protective coating around the
cells, which tends to make them smaller, less sticky, and less
subject to outside influences. This would tend to make them pass
through a filter more readily. In line with this idea also is the
well-known fact that disinfection is less efficient in cold water
than in warm water.

Another way of viewing the matter is that cold retards the growth of
bacteria on the filter, thus reducing the effect of the
_Schmutzdecke_. Still another view of the greater danger from
bacterial contamination in winter is the theory that cold prolongs
the life of the bacteria by merely preventing them from living
through their life cycle and reaching natural old age and death as
rapidly as in warm weather.

Another topic in Mr. Hardy's paper which has interested the writer
is that of preliminary filters. The experiments described at length
indicate clearly that such devices would prove of little or no
benefit under the conditions existing in Washington, and that when
the river contains considerable amounts of suspended clay nothing
less than chemical coagulation will suffice to treat the water so
that the effluent will be perfectly clear. Preliminary filters have
been used for a number of years at various places and with varying
success. In few instances have they been operated for a sufficient
length of time or been studied with sufficient care to determine
fully their economy and efficiency as compared with other possible
methods of preliminary treatment.

Among other experiments on this matter are those made at Albany, N. Y.,
and published by Wallace Greenal