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dotted line indicates the amount of carbon dioxide present, and the finer double line the amount of
organic matter.
A consideration of the carves shows that there is, as might be expected, a general correspondence
in the three evidences of contamination. The carbon dioxide and organic matter vary together
fairly uniformly, and if it were not for Observations VII. and VIII., the number of micro-organisms
would fairly vary with them. In these two observations, however, and especially in No. VII., the
number of micro-organisms is disproportionately high, and it must be inferred that bacterial contamination
of the air, though as a rule parallel to chemical contamination, may yet vary quite
independently as the result of special conditions. Wind or strong draughts for example might be
expected to raise the number of micro-organisms in the air, while lowering the degree of chemical
impurity. I cannot, however, explain the high numbers of bacteria found in Observations VII. and
VIII. in this manner, as there were no particularly strong draughts noticeable on these occasions.
On the whole the correspondence in the carves in the diagram is greater than I anticipated, but
although this is so, it is plain from the occasional discrepancies that chemical examination alone
cannot be taken as a trustworthy guide to the bacterial contamination of the air.
B.—As concerns the species of micro-organisms found.
I pass now from the total numbers of organisms found in the air to the actual species identified.
The identification of the species in so large a number of colonies was an infinitely more laborious
matter than the enumeration of the colonies, and the labour was much increased by the fact that many
of the common air organisms are imperfectly described. Indeed I met with several species to which
I could attach no name at all, and in many other cases the identification was only approximate. My
method of procedure was to make a sub-cultivation from the original colony, and if it proved, as was
occasionally the case, to be impure, to separate the different species by the usual approved methods
until I had pure cultures. These were then examined microscopically, stained and unstained, in order
to determine the group to which the species belonged. Subcultures were then made on different
media, gelatin, agar-agar, potato, broth, milk, etc., and were incubated at 20° C. and 37° C. The
colour, mode of growth and other characters of the species were thus determined, and by comparison
with the descriptions in standard text books and monographs, the species could as a rule be satisfactorily
identified. In many cases, however, further tests bad to be employed. In the case of a series of six
different yeasts, the power of fermenting grape sugar was kindly determined for me by Mr. WoodSmith.
In the case of bacilli resembling bacillus coli communis, in addition to testing the power of
coagulating milk, 1 had to test the power of gas formation in glucose-gelatin, the production of indol,
and the reduction of neutral red. Flagellum staining was resorted to in one or two cases. The bacilli
belonging to the diphtheroid group were grown on serum, and tested by Neisser's method of staining,
and their power of acid formation in glucose broth was also determined. One colony, which in some
respects resembled the diphtheria bacillus, was tested on a guinea pig, but proved quite harmless.
This was the only instance in which the inoculation test appeared to me necessary, as in no other case
(excepting the common pyogenic skin cocci and bacillus pyocyaneus) was any organism found which
resembled any known pathogenic species.

(Note.—The determination of some of the species in the foregoing list is to be taken as approximate
only, for the reasons given in the text.)
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