[BULLER & LOWE] MICRO-ORGANISMS IN THE AIR OF WINNIPEG 49 
and this half was laid on a sterilised surface. The glass wool plug, d, 
in the entrance half of the tube was then pushed by means of a 
sterilised glass ramrod into the largest of the flasks, which was again 
closed by its cotton wool stopper which had been temporarily removed. 
The exit half of the tube was then taken up, its cotton wool stoppers 
removed, and its plug of glass wool and sugar, c, pushed down into 
the medium-sized flask. The control tube was then removed from its 
test tube, its cotton wool stoppers taken out and its plug, e, pushed 
down into the small conical flask. 
By shaking the three flasks, the plugs introduced into each be- 
came disintegrated and the constituent parts uniformly distributed in 
the still fluid medium. By rotating or otherwise manipulating the 
flasks under a stream of water, the gelatine in each was caused to 
solidify and become evenly spread over their bases and sides. When 
this had been done, the flasks were labelled and set in a dark cupboard 
in a warnr laboratory, the temperature of which was usually about 
FAC. 
After three or four days’ incubation, colonies of bacteria and yeast 
and the mycelia of moulds became visible; and their number was counted 
at the end of a week. The first plug, d, taken from the entrance 
end of the experimental tube gave the number of micro-organisms 
which had been collected from the 10 litres or other quantity of air 
which had been drawn through it. The second plug, c, taken from 
the exit end of the experimental tube served as a control for the first 
plug. It was argued that if any micro-organisms had failed to be 
collected in the first plug, they would be caught in the second, if not 
in the glass wool, at any rate in the finely powdered sugar. Only on 
very rare occasions were any micro-organisms developed in the flask 
containing the second plug, c, and not once did their number exceed 
three. On the single occasion when three were obtained in this plug, 
the maximum number of micro-organisms, namely 260, were developed 
from the first plug. It is clear from this that the first plug success- 
fully arrested practically all the micro-organisms contained in the air 
drawn through it. When micro-organisms were developed from the 
second plug, their number was added to those developing from the 
first. The plug of the control tube (Fig. 2, e) served to show whether 
or not the experimental and control tubes had been properly sterilised 
in the first place, and whether or not any micro-organisms had 
settled in the mouth of the two tubes through the agency of the wind 
or air currents during the time that the cotton wool stoppers had been 
removed from the entrance ends of the two tubes. It was only ex- 
ceedingly rarely that micro-organisms developed in the flask containing 
the plug of the control tube, and on no occasion did their number 
