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13 to 10. Twice they were less than in the fresh air and twice equal; four times they were somewhat
greater and four times much greater. The differences were especially marked in the cases of the
carriages and lifts, i.e., the most crowded places examined. The platforms and passages come out
actually better than the fresh air; the tunnels, perhaps unfairly compared with the relatively pure air
of a park, are only a little worse. So far, therefore, as the total number of micro-organisms in the
air go, the Central London Railway does not compare unfavourably with the conditions known to
exist in inhabited rooms generally.
Turning now to the figures in the second and fourth columns of the preceding table, those
dealing with the micro-organisms capable of growing at the temperature of the human body, it will
be seen that the difference between the air of the railway and the fresh air is very much greater. The
railway air contains rather more than twice as many such organisms as the fresh air ; the total averages
are 6.9 and 3.4 per 5 litres respectively. The excess of such organisms over those found in the fresh
air is seen in the averages from carriages, platforms, passages, lifts and tunnels alike, and is an almost
constant feature of the individual observations as well as of the averages. Only once did the number of
colonies in the fresh air agar-agar plate incubated at 37° C. exceed the number in the corresponding
plate from the railway air; once, too, they were equal.
This fact struck me at once as perhaps of some importance and worth further investigation.
The attention which has hitherto been paid to the bacterial flora of the air is very slight; practically
all that has been attempted is an enumeration of colonies in gelatin plates. The term " psychrophil "
is sometimes used to denote bacteria whose optimum temperature for growth is about that of the
air, and which cannot grow at all at the body temperature. The term " mesophil " is used for those
organisms whose optimum temperature is about that of the human body; but most of these can also
grow, though more slowly, at air temperature. With " thermophil " bacteria we are not here
concerned. The foregoing table shows very plainly the well-established fact that the great majority
of the air bacteria belong to the psychrophil group.- I hoped, by the simple method which I
had adopted of incubating similar samples at 20° and at 37° C., that the ratio of mesophils to
psychrophils yielded by the figures obtained might give more valuable information than a bare
enumeration of colonies in gelatin plates. For pathogenic bacteria all belong to the mesophil group,
and one might expect to find bacteria derived from human sources, even if not pathogenic, to belong
chiefly to the mesophils.
In this I hope I have carefully analysed all my results, excluding the moulds altogether as
confusing the issue ; I have worked out the percentage of mesophils for all my observations, both
on fresh air and on railway air. The result was disappointing, probably because the volume of
air taken in each sample was too small, and the number of observations too few. The variations
in the mesophil percentage proved so great that I fear no reliable inferences can be drawn from them.
The figures, such as they are, are these—
Fresh air—(I exclude one observation, No. IX., because the number of mesophils
which happened to grow in the agar-agar plate actually exceeded the total in the gelatin
plate—3 and 2 respectively. With such small numbers no result of any value can be
obtained.) With this exception, and excluding moulds altogether, the average percentage of
mesophil organisms was 10.6. The extremes of the different observations were 0.0 and 37.5
per cent.
Railway air—(In observation No. IX. no comparison could be made because both
samples of air were accidentally incubated at 20° C.)
The remaining 11 observations yielded a mesophil percentage of 17.9, the extremes of the different
observations being .9 and 66.6 per cent. The difference between 10.6 and 17.9 per cent, is not a
great one, and the number of observations is too small to allow me to lay much stress on it. On
further investigation of the matter I discovered another fact which from the point of view of public
health appears to me still more to minimise the value of the mesophil percentage. This fact is the
very large number of sarcinae I found in the railway air, as will be seen later in the table of species
identified. Sarcinae are the commonest of air organisms. I identified them no less than 106 times in
the course of these experiments. For some reason, which I am not at present able to explain, they
were very much more abundant in the railway air than in the fresh air—as 87 to 19. They are
almost invariably mesophil, and they practically account for the excess of mesophils in the railway air.
Yet, so far as I am aware, we have no really sufficient grounds for attributing to them a human
origin. What their natural breeding places may be we do not know, and it would be a matter of some
interest to determine it. But we know that they are non-pathogenic, and that they are not amongst
the commonest human saprophytes. They are common, it is true, in the lungs and the stomach, but
the species found in the air are not those which are so abundant in these situations. The common
air sarcinae, sarcina lutea, sarcina flava and sarcina alba, are practically known as air organisms only.
They are at times found in cultures from the mouth and throat, but it is as likely that they get into
the throat from the air, as into the air from the throat. While, therefore, I think it possible that future
researches on the lines I have mentioned may add a new feature to the practical utility of bacteriological
air examination, I fear I cannot claim any serious importance for these limited observations on
the mesophil-ratio.
Before leaving the subject of the total number of micro-organisms in the air, it will be well to
examine the relation between these numbers and the other evidences of organic contamination. On
every occasion on which I took samples, Mr. Dick, for the chemist to the Council, took samples for
chemical examination. I am therefore able to give figures enabling this comparison to be made,
Dr. Clowes having been so good as to furnish me with his results as to the carbon dioxide and organic
matter present in the air. I have embodied our joint results in a diagram from which the variations
in the three evidences of air contamination can be more readily appreciated than from tables of
figures. The observations have been arranged in their chronological order, without any classification
as to the source of the samples, as this renders the variations more sudden and striking. The thick,
single line indicates the number of micro-organisms, including moulds, found in 5 litres of air. The