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The authorities just quoted make the following remarks as to the sources of the several impurities
in sewer air.
Carbonic acid in sewer air over and above that in fresh air may have two sources. It may in
part be due to diffusion into the sewer from the neighbouring soil, but probably its chief source is
oxidation of organic matter in the sewage and the air of the sewer.
The source of the organic matter in sewer air over and above that present in fresh air at the same
time is of course the sewage itself.
The organic matter arising from the sewage is probably wholly, or for the most part gaseous, and
the gaseous organic matter will probably be of two kinds, that volatile per se, and that volatile with
aqueous vapour from the sewage water.
Organic matter may also get into the air where splashing occurs from the entry of a side drain
high up in the wall of a sewer. There are two possible sources of the majority of micro-organisms in
sewer air. They may in the first place be derived from the sewage and sewer walls, or secondly from
fresh air gaining access to the sewer by ventilation. Were their source the sewage or the walls of the
sewer, their number would increase with the length of sewer traversed by the air or with the length of
time during which the air was in contact with the sources of contamination. They found, however,
that the very opposite was the case, the number of micro-organisms diminishing with the distance the
air had travelled from the nearest ventilator. This fact is not at all surprising when we remember the
very damp condition of the sewer walls and the constant tendency of organisms to deposit. It has been
abundantly proved by Naegeli and other observers that liquids and damp surfaces do not, with ordinary
air currents, give off micro-organisms to the surrounding atmosphere. It may be interesting to note
here that I found only moulds and micrococci in the water streaming down the roof of the sewer.
A decrease in the number of micro-organisms in fresh air was followed by a decrease in the number of
micro-organisms in sewer air. An additional argument may be derived from the fact that the average
proportion of moulds to bacteria was nearly the same in the sewer air and corresponding fresh air.
Although the authors did not make any use of the microscope to determine whether the various colonies
derived from sewer air were similar to those derived from corresponding fresh air, they clearly state that,
so far as naked eye observation went, the colonies with one exception were identical.
For the purposes of my own investigation a stretch of sewer was chosen in which not only were
the conditions simple and uniform, but also of long standing. It was thus possible to introduce a known
complication, and to estimate its amount and effect upon conditions already known. For instance, it
was easy to arrest the passage of air in and out of the sewer at a known point, as well as to causo
a degree of splashing sufficient to disseminate in the air of the sewer any particles existing in the sewage
itself that were capable of being disseminated.
The sewer actually chosen is known as the King's Scholars' Pond sewer. It runs in a straight
line under the Green-park, from Piccadilly in the north to Buckingham Palace-road in the south. It
was constructed some 120 years ago, and has therefore been in use long enough to become itself
contaminated in every possible way. In length it is about 300 yards, it is barrel shaped, and about
11 ft. high by 9 ft. wide. It is ventilated by open gratings only in the crown of the sewer. These are
placed in tbe turf at distances of about 50 yards apart, so that the air circulates over them with perfect
freedom.
There are two entrances, one close to Piccadilly, and the other about 200 yards further on in the
Green-park. One important feature is that no lateral sewers or house drains enter it from one end to
the other, the consequence is that it contains an even flow of sewage together with a thorough free access
to the air of the park.
So far as regards the enquiry into the micro-organisms in sewer air, it is obvious that it was
possible to estimate under the most favourable conditions, with fair accuracy, the number and character
of the organisms in the air of this particular sewer as compared with the number and character of microorganisms
in the air above.
The amount of moisture, carbonic acid, and the number of micro-organisms was estimated in the
outside air, and then in the air of the sewer immediately afterwards. The organic matter was not
estimated, as the data furnished by the permanganate and other processes at present in use is not very
reliable. In addition to these determinations, the barometric pressure, direction and approximate force
of the wind, shade temperature, the temperature of the sewer air, rate of the air current, and quantity of
sewage were also noted.
The methods employed for determining the water, carbonic acid, and micro-organisms, were
Haldane and Pembrey's* for water and carbonic acid, and Frankland'st for micro-organisms. The
experiments may be conveniently divided into three series.
I. Normal sewer air experiments.
II. Splashing experiments.
III. Deodorising experiments.
For the convenience of working, a wooden staging was erected across the sewer a few feet from
the second entrance, by which I always gained access to the sewer.
A ventilator occurs at a very short distance above the staging; this ventilator was closed during
all the experiments with the exception of the first. All the air gaining access to the sewer had
therefore to travel 40 to 60 yards either way before reaching the apparatus. The estimations of microorganisms
both in fresh air and in sewer air were made in duplicate with one exception.
The following tables give the results of the three series of experiments, Table I. of the normal
sewer air experiments, Table II. of the splashing experiments, and Table III. of the deodorising
experiments.
* "Philosophical Magazine," 1890.
† "Philosophical Transactions," 1887, page 113.