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of the examination of the 6-foot secondary coke-bed effluents are too few in number to allow of
useful conclusions being drawn, but here also the same agreement occurred. The above must not
be taken as meaning that the percentage deviation from the mean showed any parallelism as
regards the crude sewage and corresponding effluents.
So far as a diminution in the number of bacteria may be taken as an indication of purification,
it must be admitted that the above results are not satisfactory. And if they are unfavourable
from the point of view of percentage reduction effected, still less are they favourable when
judged by the actual state of the effluents.
It is noteworthy that the reduction in the number of bacteria was much greater in the
effluent from the 4-foot coke-bed than in the effluent from the 6-foot primary coke-bed, and was
nearly as much as in the effluent from the 6-foot secondary coke-bed.
Possibly the records as regards the 6-foot primary coke-bed indicate a retrogade change.
Since the coke-beds are intended to encourage the life processes of bacteria it might be
urged that an increase, or, at all events, the absence of a marked decrease in the number of
bacteria in the effluents is of little importance. Further, that it may even be a desirable thing
that an effluent only partially purified should carry with it the bacteria which have been engaged
in the work of purification in order to complete their work. There may be truth in these contentions,
but as experience has shown that the bacterial beds which yield the best chemical results
yield also the best results bacteriologically, and as river water already contains all the necessary
germs of putrefaction and nitrification, it would seem to be undesirable to discharge with an
effluent countless germs of a kind to be associated with the recent evacuations of animals. Moreover,
some of these micro-organisms are pathogenic, and doubtless many of them, although perhaps
slowly working in the direction of purification, are, in a sense, intruders, since the work could
probably be more efficiently carried out in their absence by other and unobjectionable bacteria.
As regards the value to be assigned to the estimation of the total number of microbes, it
mav be said that the greater the number of micro-organisms in a liquid containing a mixed
bacterial flora the more likely is it that some of them are of a harmful nature. But further than this,
the prolonged investigations carried out in this enquiry as regards the species of microbes and
their relative numbers give to the above figures a special and peculiar value. For while there is
no fixed proportion between the total number of bacteria, and, for example, the number of proteuslike
germs, of B. coli and of spores of B. enteritidis sporogenes, it has been abundantly shown that
with a sample of effluent or raw seivage, containing an average number of microbes, say three to nine
millions per c.c., the number of " sewage proteus " of B. coli and of spores of B. enteritidis sporogenes is
likely to be not less than 100,000 ; at least 100,000, and usually more than 100 respectively. Further,
that more than one-tenth of the bacteria are likely to be " liquefiers," and only about one in 20,000
present as spores (aerobes).
As the bacterial composition of London crude sewage does not differ widely from the raw
sewage of most other towns, if is believed that these records have a wide range of usefulness At
the risk of repetition it may be pointed out that their bearing on the bacterioscopic examination
of potable waters is very important.