[BULLER & LOWE] MICRO-ORGANISMS IN THE AIR OF WINNIPEG 45 
Before an experiment was to take place, the aspirator was filled 
up with water, set upright on its stand, and the level of the water 
allowed to descend to the first circular marking. Since nearly half 
of the experiments had to be carried out when the temperature of the 
air was below freezing point, and many when the temperature was 
below zero Fahrenheit, it was necessary under these conditions to fill 
the aspirator with warm water. 
Owing to the extreme paucity of micro-organisms present in the 
air during the winter, it was found advisable very frequently to aspirate 
from 20 to 100 litres of air through the experimental tube, instead of 
merely 10 litres. When more than 10 litres was to be aspirated, a 
second aspirator, similar to the first, was prepared. As soon as 10 litres 
had been aspirated by the first aspirator, the stopcock, s, was closed 
and the stopper of the stopcock, s', was pulled out. The glass tube 
passing through the stopper at the top of the aspirator was withdrawn 
and the second aspirator substituted for the first, and connected in a 
similar mannner. The water in the fresh aspirator was allowed to des- 
cend to the first circular marking. The stopper of the stopcock, s', 
was then replaced, and when both of the glass stopcocks had been opened 
and the clamp from the exit tube had been removed, a further 10 litres 
could be aspirated through the experimental tube. The aspiration of 
100 litres of air involved nine changes of the aspirators. Each change 
was carried out in about half a minute. The aspiration of 10 litres 
was accomplished in 12 minutes. 
The flexible lead pipe, which was ten feet long and which had an 
internal diameter of a quarter of an inch, was connected with the ex- 
perimental tube and the aspirator by short lengths of rubber tubing. 
The object in using the lead pipe was to allow of the air being aspirated 
through the experimental tube at a distance of about ten feet from the 
aspirator, which it was necessary for the experimenter to manipulate. 
The apparatus was so arranged during the aspiration of the air 
that the wind always moved in a direction parallel to the axis of the 
experimental tube and from its exit to its entrance end. The direction 
of the wind is indicated in the figure by means of an arrow. The lead 
pipe was placed at right angles to the direction of the wind, so that 
any micro-organisms disturbed by the manipulation of the aspirator 
could not pass to the mouth of the experimental tube. The advan- 
tage of using tubing made of lead rather than of glass or rubber lies 
in the fact that a lead tube is sufficiently flexible but, at the same time, 
not readily cracked or broken. 
A series of volumetric experiments was made at St. Charles, at a 
distance of six miles from the city of Winnipeg. Here, on the open 
prairie, it was not found convenient to use the aspirator, owing to the 
