[BULLER & LOWE] MICRO-ORGANISMS IN THE AIR OF WINNIPEG 55 
the first week in April, was 19.5. The average number from the end 
of the first week in April to the end of the first week in November was 
1,206.6. The highest monthly average was in October, and the next 
highest in April. Both of these averages coincided with the windiest 
periods of the year. The minimum number of micro-organisms falling 
per square foot per minute, namely 3, was observed on two occasions, 
January 7th and March 25th respectively. The next lowest record, 
~ 
of 3.5, was for February 12th. On February 19th the number was 5. 
We may now discuss the effect of the weather on the results. The 
variation in the number of micro-organisms in the air is due, as has 
been explained by many other observers, to changes in temperature, 
alteration in the velocity of the wind, and to the fall of rain and snow. 
An inspection of the volumetric results plotted in Plate I, shows that 
temperature has the most influence, for the monthly averages are 
lowest during the cold winter months, rise steadily during the spring, 
and are highest during the hot summer months. Showers of rain or 
snow temporarily clean the air by carrying much of the suspended dust 
to the ground. This is shown by the following observations. On 
August 31st, after two rainy days, 12.5 micro-organisms were con- 
tained in 10 litres of air, whereas during the previous week, when the 
velocity of the wind and the temperature were almost exactly the 
same, the number was found to be 137. The plate method also showed 
a fall in the number for the same period. On November 10th, after 
two days’ snow, 24 litres of air did not yield a single colony, and only 
36 micro-organisms fell per square foot per minute; whereas a week 
earlier 27 micro-organisms occurred in 10 litres, and 399 fell per square 
foot per minute. The influence of the wind factor became obvious 
on several occasions. With an increase in the wind velocity, the num- 
ber of micro-organisms usually increased, and with a decrease in wind 
velocity the number of micro-organisms in the air decreased; thus, on 
April 13th, 1908, there were 30 in 10 litres during a gale, whereas on 
April 6th, when the wind was only strong, there were 6, and on April 
22nd, when the wind was slight, there were 22.5. The effect of wind 
on the number of micro-organisms falling per square foot per minute 
is very obvious; thus, during three gales, on April 10th, April 13th, 
and on October 21st, the enormous totals of 4,180, 8,506 and 8,440 
were obtained respectively. 
The nature of the Micro-organisms.—The micro-organisms which 
developed in the cultures were included in the three groups, Bacteria, 
Yeasts and Fungi. Usually the bacteria predominated; but this was 
not always the case. Thus on October 6th a pink yeast was a dominant 
form, there being four yeast colonies to three of other micro-organisms. 
This yeast came up in the cultures almost every week in the year, 
