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In the preceding table I have enumerated all the species of bacteria, streptothrices and yeasts
which I could certainly or approximately identify, and indicated the number of colonies in the railway
air and in the fresh air which were referred to each species. The total number of colonies from which
cultures were made was between 500 and 600. The number of species identified was 58, viz.,
26 micrococci, 24 bacilli, 3 streptothrices and 5 yeasts. In addition to these there were some 10 or 12
species which baffled my efforts at determination, and of which most have probably not been described.
It will be seen that I have omitted moulds from the table. By far the commonest mould met with
was penicillium glaucum, and not far behind it came another penicillium, closely allied, but of a more
ashy-grey tint. The genus aspergillus was also common, the usual species being aspergillus glaucus,
though aspergillus flavus, aspergillus fumigatus, aspergillus niger and other species were sometimes found.
The genus mucor was much less common. In addition to these genera were a number which I could
not determine. I made many sub-cultures, and examined them in various ways, but I abandoned the
attempt to name them, partly because I could not find any work in which they are adequately
described, and partly because it did not appear of any great importance to name them. As regards
total numbers, I found moulds somewhat more abundant in the fresh air than in that of the Central
London Railway. My figures show 107 mould colonies from the fresh air as against 94 from that of
the railway. As regards the proportion of moulds to bacteria, I find that my results give a much
lower proportion of moulds than that usually accepted as present in the air. Taking the totals from
the gelatin plates incubated at 20° C., I found in the railway air, out of a total of 574 colonies,
93 moulds, or 16.2 per cent. In the fresh air, out of a total of 407 colonies, 101 were moulds, or
24.8 per cent. The highest proportion of moulds was met with in a sample of fresh air from Hydepark,
where it reached 76.4 per cent, of the total, while in the two samples taken from a scaffold in
Newgate-street it reached 41.3 per cent. In the air of the Central London Railway the proportion
twice reached 44.4 per cent., and was once as low as 1.8 per cent. The comparatively low average
percentage of moulds may be due to the fact that I habitually made my final count on the fourth day
of incubation for reasons which I have already given. As regards the species of moulds found
respectively in the air of the Central London Railway and in the fresh air, I have only to record that
the majority of the species which I was unable to identify were found in the fresh air. The species
of penicillium, aspergillus and mucor were distributed more or less indiscriminately.
As regards the bacteria, the most striking fact brought out in the above table is the very great
relative preponderance of sarcinae in the air of the railway, to which I have already alluded, and
which I am unable to explain. I am not aware of any published statistics dealing with the relative
numbers of different bacterial species in pure and in vitiated air; so far as I know, no such observations
have been previously made. The fact, therefore, must for the present stand alone. It may
prove of some value as a test of atmospheric contamination, or it may not. But the mere presence of
large numbers of these non-pathogenic sarcinae can hardly be, in itself, harmful.
Much the same criticism may be applied to the lesser, but still marked preponderance, in the
railway air of the organisms which I have designated staphylococcus cereus flavus, staphylococcus
cereus albus and micrococcus flavus. These organisms are closely related to the sarcinae, and differ
from them mainly in their inability to form the typical bales and packets which characterise the genus
sarcina. This property varies, however, on different media, and it is quite possible that some of the
colonies I have tabulated under the above headings were really sarcinae. Indeed, I was able to
correct several errors of this sort.
Attention may further be drawn to the genus streptothrix, which occurred in the railway air
between twice and thrice as frequently as in the fresh air.
Little remains to be said of the other species found, which, for the most part, occurred in small
numbers. By far the greater number were present indiscriminately in the railway air and in the fresh
air. Eleven species were found in the railway air only, and eight in the fresh air only.
I was struck by the chemical inactivity of the vast majority ot the micro-organisms present in
the air. Of the various yeasts tested only one species could ferment sugar, and of a large number of
bacterial colonies which I had occasion to test I did not find one which yielded gas in glucose-gelatin
shake-cultures, or which was capable of causing the lactic acid fermentation in milk.
A word of comment and criticism is required in conclusion as to my failure to demonstrate
pathogenic species in the air. My attention was so constantly directed to their discovery that I feel
tolerably sure that none were overlooked in the 600 or so colonies which I examined. The absence of
streptococci was a surprise to me in view of their abundance in the human mouth. The solitary
species I found was in the fresh air in Newgate-street; it formed no acid and did not coagulate milk.
The white pyogenic coccus (staphylococcus pyogenes albus) was found twice, and the allied staphylococcus
epidermidis albus four times. These are common on the human skin, and may well have had
a human, or at least an animal origin. Bacillus pyocyaneus, rarely pathogenic to man, was once found
in a passage of the Central London Railway. I have classed as bacillus pseudo-diphtheriae two, if not
three, species of diphtheroid bacilli, all of which were submitted to the usual tests. They were not the
diphtheria bacillus, nor were they Hoffmann's bacillus nor bacillus xerosis, so I have been content to
class them vaguely. The xerosis bacillus, which may have had a human origin, was met with five times.
Bacillus coli communis, which I confidently expected to meet with from its abundance in horse-dung
—so common an ingredient in street dust—was never once found, though every colony at all
resembling it was carefully tested.
The probable explanation of this approximate failure to find not only pathogenic organisms but
even the commoner saprophytic organisms of the mouth and intestine is, in my opinion, to be found in
the method which I was obliged to employ. In order to compare different samples of air as to bacterial
contamination, the obvious and only method was to determine the number and the varieties of the
bacteria present in known volumes of air. The task would have been impossible had I not employed
a very limited volume for each estimation—in this case 10 litres for each sample examined. The
primary problem was in essence a botanical one, and I do not regret the method I used, because the