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respectively 216 and 330 colonies. Six similar plates, inoculated each with 1 c.c. of the sewage
diluted 100,000 times, yielded respectively 21, 39, 26, 27, 28, and 29 colonies. The total average is
given above. The same sewage, however, after being kept for three days in a stoppered bottle, showed
an enormous increase in bacteria, a similar plate inoculated with 1 c.c. of the sewage diluted 100,000
times yielded 338 colonies, representing 33,800.000 bacteria per cubic centimetre.
From the Fleet sewer in Snow-hill the results were somewhat higher. The sewage was taken
on March 2nd, at 11.30 a.m., and cultivations were made immediately. Three agar-agar plates, made
in the same way as the above, with 1 c.c. each of the sewage diluted 100,000 times, gave respectively
40, 28, and 34 colonies, yielding an average of 3,400,000 bacteria per cubic centimetre. With this
sewage also we made further experiments, to ascertain what proportion of the bacteria present would
grow at a temperature of 37°C., or with the addition of .05 per cent. carbolic acid, or under the influence
of both these conditions combined. Two agar-agar plates, inoculated each with a cubic centimetre
of the sewage diluted 10,000 times, with the addition of .05 per cent. carbolic acid, and incubated
at 37° C. yielded respectively 59 and 38 colonies, giving an average of 485,000 organisms per cubic
centimetre which could grow under these unfavourable conditions. Two plates made in a similar
manner, without the addition of carbolic acid, but incubated at 37° C., yielded respectively 103, and
78 colonies, or an average of 905,000 colonies per c.c., which could grow at this temperature. A single
plate inoculated with 1 c.c. of a dilution of 1 in 100,000, with the addition of .05 per cent. carbolic
acid, but incubated at 22° C., yielded 19 colonies, or 1,900,000 organisms per c.c. of sewage. These
results indicate that a temperature of 37° C. inhibited the growth of 73.4 per cent. of the total number
of organisms present in this sewage, while the addition of .05 per cent. carbolic acid inhibited the
growth of 44.2 per cent. of the total number ; the combination of .05 per cent. carbolic acid, with a
temperature of 37° C., inhibited the growth of no less than 85.8 per cent. of the bacteria present.
In the case of the sewage collected at Barking, at 11.30 a.m., on April 4tli, an hour and a-half
elapsed before cultivations could be made in the laboratory. Two agar-agar plates, made each with
1 c.c. of the sewage diluted 100,000 times, and incubated at 22° C., yielded respectively 21 and 17
colonies, or an average of 1,900,000 bacteria per c.c. of sewage. A gelatine plate, however, made in
the same way gave 64 colonies, or 6,400,000 bacteria per c.c. of sewage. It is known that gelatine
is a more favourable medium for the growth of bacteria than agar-agar, but the discrepancy between
these two results is surprising; it is easy, however, to attach too much importance to isolated observations
of this kind. The number of bacteria calculated from the agar-agar plates is less than might
have been expected, but the results given by similar plates kept at a higher temperature, or with the
addition of '05 per cent, carbolic acid, are proportionate. With the same sewage at the same time, two
similar agar-agar plates (each containing 1 c.c. of the sewage diluted 10,000 times) were inoculated and
incubated at 37°C.; one yielded 27 and the other 60 colonies, or an average of 435,000 bacteria per c.c. of
sewage, able to grow at the higher temperature : i.e., 77"2 per cent, were inhibited in their growth. Two
agar-agar plates containing each 1 c.c. of the sewage diluted 100,000 times, but with the addition of .05
per cent. carbolic acid, yielded when incubated at 22° C., 11 and 8 colonies respectively, equal to 950,000
organisms per c.c. of sewage able to grow in the presence of this amount of carbolic acid : i.e., 50 per
cent. were inhibited in their growth. Lastly, the two conditions were combined and two agar-agar
plates were inoculated with 1 c.c. each of the sewage diluted 10,000 times, and incubated at 37° C.
with the addition of .05 per cent. carbolic acid; these yielded respectively 15 and 8 colonies, or an
average of 115,000 organisms per c.c. of sewage able to grow at 37° C. with the addition of the above
percentage of carbolic acid : i.e., 94 per cent. were inhibited.
With regard to the total number of organisms present in sewage the highest results were
obtained from a sample taken at the Crossness outfall at 2.30 p.m. on July 10th. This is doubtless
attributable on the one hand to the warmer weather then prevailing, and on the other to the fact that
three hours elapsed between the time the sewage was collected and the making of the cultivations.
The possible effects of this interval were however obviated as far as possible by keeping the bottle of
sewage during its transit packed in a chloride of ammonium freezing mixture. Six agar-agar plates
were made, each containing 1 c.c. of the sewage diluted 100,000 times, and they were incubated at
22° C. They yielded respectively 124, 127, 105, 119, 110 and 88 colonies, or an average of 11, 216,
666 micro-organisms per c.c. of sewage.
The sewage collected from the fever hospital at Homerton on May 23rd at 11 a.m. was examined
chiefly for the purpose of discovering the typhoid fever bacillus. Of two agar-agar plates made with
1 c.c. of the sewage diluted 100,000 times, and incubated at 22° 0, one gave 45 and the other 35
colonies, or an average of 4,000,000 bacteria per c.c. of sewage. Two gelatine plates made at the
same time and of the same degree of dilution yielded after two days 41 and 42 colonies, or 4,150,000
bacteria per c.c.'of which 2,600,000 liquefied gelatine. But many more colonies would have appeared
but for the rapid liquefaction of the gelatine, so that this is not to be regarded as a result fairly
comparable with that of the agar-agar plates.
A similar experiment with agar-agar plates, made with sewage taken from the Bridge-street
sewer at Homerton on June 21st, at 2.30 p.m., gave an average of 4,050,000 bacteria per c.c. of sewage.
It will thus be seen that very wide variations exist in the total number of micro-organisms
present in sewage at different times and in different places, as indeed might safely have been predicted.
Temperature is one important factor in determining the rapidity of their reproduction, and hence the
increase in their numbers ; dilution of the sewage by rainfall must also exert a marked influence. The
estimations on which the foregoing figures are based were made with all possible care, and we believe
them to be as accurate as possible for each particular sewage at the time we examined it. But it is
clear that a small number of determinations, such as we have made, can afford only an indication of
the average number of bacteria present in sewage, nor was it our desire to pursue this subject in
detail, as we were concerned with the nature of the organisms present rather than with their actual
numbers.