50 ROYAI, SOCIETY OF CANADA 
exceed two. When an occasional micro-organism did develop, the con- 
tamination could be traced to a change in the direction of the wind 
during the exposure of the tubes in the field. When micro-organisms 
developed from this control plug, their number was counted and an 
equal number was deducted from the number which developed in the 
flask containing the first plug (d) of the experimental tube. 
The Plate Method.—Shallow Petri dishes, five and a half inches 
wide and half an inch high, were sterilised and then partially filled with 
the culture medium in the usual manner. When this had solidified, 
the plates were enclosed in a sterilised glass vessel, to protect them 
during their transportation to the field. When the plates had been 
brought to the place of exposure, they were set on a horizontal support 
raised from three to six inches above the level of the ground which, 
it may be remarked, is exceedingly flat everywhere in the neighbour- 
hood of Winnipeg. The lids of the Petri dishes were raised and the 
surface of the gelatine exposed to the atmosphere for one or more 
minutes. The time of exposure varied considerably: at those times 
when micro-organisms were abundant, 7.e., during the summer, and 
when the wind was strong, it was only one minute in length; but after 
heavy rain and during the winter months, when the air was almost 
sterile, owing to the exceeding paucity of the micro-organisms present, 
it was found necessary, in order to collect any micro-organisms 
at all, to increase the time of exposure to 5 minutes, 10 minutes, or, on 
some occasions, to one hour. After the exposure, the lid of the Petri 
dish was replaced; the closed lid was again covered with the sterilised 
glass vessel and then taken back to the laboratory. The Petri dishes 
were kept in the same room, and therefore subjected to practically 
the same conditions, as the flasks containing the plugs. 
The counting of the colonies and fungus mycelia was usually carried 
out on the fourth day after making the cultures. It was always found 
that micro-organisms on the surface of the gelatine developed more 
rapidly than those which were embedded in the gelatine in the flasks. 
Doubtless, this difference in the rate of development is to be accounted 
for by the difference in the accessibility to oxygen for the micro- 
organisms in the two cases. In order to count the number of colonies, 
etc., which had developed, the plate was placed on a piece of black 
paper divided into small areas by white lines. The number in each area 
was counted in succession. The total was thus obtained without any 
danger of any colonies, etc., being overlooked or counted twice. The 
unit of measurement fixed upon for the rate of fall of micro-organisms 
was the number falling on one square foot per minute, and the results 
of the plate records have been tabulated according to this standard. 
The actual area of the gelatine exposed was 20.95 square inches. 
