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Conclusions.
A.—As concerns the total number of micro-organisms found.
Owing to the fact that a large majority of the organisms present in the air do not grow at the
body temperature, the totals must be taken from the gelatin plates incubated at the room temperature.
The true totals must be higher than those indicated in the table, for not only did I fix the arbitrary
limit of four days for counting the colonies, but many organisms were doubtless present, incapable of
growing in the media employed. The numbers which I found agree fairly with those previously found
by other observers, though they are somewhat higher than those which have commonly been recorded.
Differences in method, and especially in the date at which the colonies were counted, may explain this.
As regards the number of micro-organisms in the fresh air of London, I may quote the figures given
by Mr. J. Parry Laws in his report on sewer air investigations to the London County Council in 1893.
He found in August, September and October an average of 25.8 organisms in 5 litres of air, the
number varying from 11 to 59.5. But in January, February and March he found an average of only
57 organisms per 5 litres, his highest count being 9.5. My higher average of 33.9 is probably chiefly
due to the fact that it includes several samples taken in crowded thoroughfares.
The highest number of organisms I found in fresh air was at the outlet of the lifts at Shepherd'sbush
station (93 in 5 litres). The next highest (60) was in Newgate-street on a rather windy day,
followed by Oxford-street (45), and the steps of the Royal Exchange (44). My lowest figures were
obtained in the quiet parts of Kensington-gardens and Hyde-park, where the number once sank as low
as 2 organisms in 5 litres. The figures are in fact in complete general accord with the well-established
principles that micro-organisms abound in the air in proportion to the amount of traffic going on in
the vicinity, that they are increased by wind and diminished by rain.
Turning now to the air of the Central London Railway, it is apparent that the same general
principle holds good. In the foregoing table I have classified the results in accordance with the places
where the samples were collected. The highest averages are found in the railway carriages, closely
followed by the platforms and lifts (63.6, 60 and 56 organisms per 5 litres, respectively). In the
passages and tunnels the numbers sink to 20 and 17. There is a direct correspondence between the
amount and concentration of the human traffic and the number of micro-organisms in the air.
It will be seen that I found the absolute number of micro-organisms present in the air of the
Central London Railway greater, on the average, than in the fresh air, but not much greater—only as.
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