54 ROYAL SOCIETY OF CANADA 
The results of the volumetric experiments have been plotted in 
Plate 1. The thick continuous line joins the points indicating the 
results of each experiment. A second line gives the monthly averages. 
Two other lines give the temperature at the time of each experiment 
and the monthly average temperatures respectively. The results of 
the plate method are given in Plate II. The thick continuous line 
joins the points of each individual experiment. A second line gives 
the monthly averages. Two other lines give the temperatures at the 
time of each experiment and the monthly average temperatures 
respectively. 
An examination of the two plates shows that during the winter 
months, 7.e., including the period from about the end of the first week 
of November to the end of the first week in April, the air in Winnipeg 
is remarkably free from micro-organisms. The average number in 
10 litres during these five winter months is less than one. During the 
same period in England, as is shown by Frankland’s curve,' the average 
number in 10 litres is about 15. The paucity of micro-organisms in 
the air of Winnipeg during the winter is accounted for by the fact that 
frost is continuous and therefore prevents micro-organisms from multi- 
plying, also that the successive falls of snow cover up the dust on the 
ground and prevent its being carried into the air by the wind. 
During what may be roughly called the summer half of the year, 
t.e., from about the end of the first week in April to the end of the 
first week in November, the average number of micro-organisms in 
10 litres was found to be 40.1. The number remains fairly constant 
during the five winter months, commences to rise in March, increases 
steadily until the middle of September, and then declines rapidly with 
the advent of early frosts. 
The maximum number of micro-organisms in 10 litres of air, 
namely 263, was observed on September 28th, 1908. The next highest 
was 137, on August 24th, and the third highest 100, on September 21st. 
The minimum number was found on March 25th, 1909, when 50 
litres of air did not yield a single colony. Other low records were as 
follow: on February 19th, when one Pencillium plant was obtained 
from 75 litres of air; on December 15th. when one Penicillium plant was 
obtained from 55 litres; and on December 29th, 1908, and January 
7th and 14th, 1909, when on each occasion two micro-olganisms were 
found in 100 litres. 
The average number of micro-organisms falling per square foot 
per minute, from the end of the first week of November to the end of 
1 P. F. Frankland. Further Experiments on the Distribution of Micro-organ- 
isms in Air (by Hesse’s Method). Proc. of the Roy. Soc., Vol. 42, 1887, Pl. 3. 
