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ratio between the bacilli and micrococci; in the sewage taken from Snow-hill and Barking the bacilli
showed a distinct numerical preponderance. This result again is in harmony with that obtained by
other observers, indeed the results we have obtained show a somewhat smaller preponderance of bacilli
over cocci than has been recorded by others. Jordan states that the comparative absence of micrococci
from sewage is a striking and highly remarkable circumstance. In any case the facts again come out
in striking confirmation of the view that the organisms present in sewer air are not derived to any
essential extent from the sewage, or they would show to some degree the same relative ratio of
micrococci to bacilli. The argument receives also important confirmation from other considerations.
A large proportion of the bacteria of sewage have the property of very rapidly liquefying the nutrient
gelatine used as a cultural medium, so large a proportion indeed as to make gelatine an impossible
medium to employ in estimating their numbers. In sewer air, on the contrary, organisms rapidly
liquefying gelatine were found to be practically absent, a fact impossible to explain on the assumption
that sewer air is able to take up bacteria from sewage.
Again the number of micro-organisms existing in sewer air appears to be entirely dependent
upon the number of micro-organisms existing in the fresh air at the same time and in the same
vicinity. With the advance of the colder weather, and consequent rapid decrease in the number of
micro-organisms in fresh air we find a corresponding decrease in the number of the micro-organisms
of sewer air, although the temperature of the sewer air and sewage suffer but a comparatively slight
variation.
If the organisms existing in sewer air were derived from those existing in sewage, then the flora
of sewer air should bear a very close resemblance to the flora of sewage. When, however, we compare
the organisms which have hitherto been isolated from sewer air with those species which we have
found to be 'predominant in sewage, it is at once evident that they bear no resemblance whatever to one
another, indeed we may go even further and state that so far as we are aware not a single colony of any
of those species which we have found predominant in sewage has been isolated from sewer air. We
consider therefore that the study of the sewage bacteria, on which we have been engaged, fully
confirms the conclusion previously arrived at from the study of the micro-organisms of sewer air, viz.,
that there is no relationship between the organisms of sewer air and sewage.
It is possible that some of the ill effects which have been erroneously ascribed to sewer air may
be due to subsoil air derived from soil polluted by constant infiltration of excremental matter through
a leaky drain. It is a well recognised fact that subsoil air does at times gain access to our dwellings,
either through the pressure of the wind on the surface of the ground or from currents induced by wide
differences between the exterior and interior temperatures. Under such conditions it is possible that
sewage may gradually extend through a permeable soil until its outer margin becomes sufficiently dry
to give off micro-organisms to the subsoil air. Whatever the danger arising from this cause may be,
it would in all probability be strictly limited in its effect.
PART II.
typhoid fever in its relation to sewage.
The sources of the micro-organisms in London sewage are clearly diverse and manifold. It
must contain species derived from the water of the different companies which supply the metropolis;
these are numerous and varied, and so far they have not been sufficiently studied. It must contain
species derived from the air, species again of which our knowledge is very limited. Further the
superficial layers of the soil must add their quota of species brought down by surface water drainage,
and the amount of organic matter in the soil of an old city must render that number a large one.
Lastly the various putrescible organic matters which constitute the essential part of sewage will add a
vast number of bacteria to the fluid; human excreta for example teem with organisms as do the waste
and household refuse passing down into the sewers from houses and manufactories.
The ultimate bacterial flora of sewage is only in part determined by the organisms which pass
into that fluid with its various constituents. Probably only a small proportion of these ultimately
survive. For some species sewage forms a favourable nutrient medium, others appear speedily to
perish in it. Those again which may be just able to grow in it under favourable circumstances, will
be liable to be crowded out in the struggle for existence by their more vigorous competitors. The
changes moreover induced in the sewage by the growth of certain bacteria, changes mostly of a fermentative
and putrefactive character, will of necessity lead to alterations in its capacity for serving as
a nutrient medium for some species; as its chemical composition alters some species may be favoured,
others arrested in their growth. The struggle for existence, the survival of the fittest, will necessarily
occur in miniature amongst these lowly organisms in sewage as elsewhere in the organic world. And
thus the flora of sewage at the Barking and Crossness outfalls comes to be, as we have found it, very
different from that of the perfectly fresh fluid as originally delivered into the drains.
It becomes, therefore, a matter of very great importance to determine the fate of certain
pathogenic organisms in sewage. Amongst the diseases which have been attributed to sewer air, and
to contamination of drinking water by sewage, two have especially attracted attention—cholera
and typhoid fever, and the spread of diphtheria is held by some to be due to the same cause.
During the period of our observations cholera was absent from London; Koch's comma
bacillus could not have been expected to be present, and though all colonies resembling it
were closely scrutinised, it was not found. The spread of diphtheria by sewage is at least a matter
of doubt, and though sewer air has been held responsible for certain outbreaks of the disease, it seems
clear that but a small part in the dissemination of the disease can be played by such a cause. The
diphtheria bacillus can be easily recognised and its pathogenic power tested; yet though a careful
look-out was kept for this organism we never found it. Typhoid fever stands on a different
footing; there is no question that the specific poison of the disease, believed on very good