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where the bread was being made,, and one on a shelf near the
ceiling. After exposure for 30 minutes the plates were re-covered
and incubated at blood heat (37° C.), for exactly 22 hours. All
the plates then showed abundant growth. Doubtless if the
plates had been incubated for 48 hours or three or four days there
would have been a vastly greater growth of colonies, and it is
probable also that if some of the plates had been placed at room
temperature certain bacteria would have grown which did not
appear at blood heat in 22 hours. It is not suggested for one
moment that these plates provide an adequate record of all the
bacteria present in the air of these Bakehouses; to obtain such
a record would have entailed an extensive investigation.* Our
object was merely to obtain a comparative idea of the air of
underground Bakehouses and above-ground Bakehouses in
Finsbury. Accordingly, the whole of the 30 plates used in this
examination were treated exactly the same in every way, the
medium, exposure, and temperature and period of incubation were
all precisely similar. The results, therefore, whilst of little value
as a complete examination of the air, are useful and reliable
for comparison with each other.
Without entering into the question of the kind of bacteria
found,* we may here insert the results respecting the number of
bacteria falling upon these plates (9.6 inches in area) during the
30 minutes:—
¤ I am aware, of course, that this gravity method of examination is
open to criticism, and in some ways not so accurate in numerical
estimation as the aspiration methods of Petri, Hesse, Frankland,
Miquel, &c. But under all the circumstances it was considered the best
in this particular case. For comparative purposes it is, of course, as useful
as any other method.
¤ Subculture of the different colonies were made in various media and
a number of micro-organisms isolated in pure culture, including a variety
of bacteria commonly found in polluted atmospheres, yeasts, moulds, and
several chromogenic bacilli. The plates on the shelves were found to
yield a number of liquefying organisms. No disease-producing organisms
were isolated.