150 BACTERIOLOGY. 
in the concavity of the slide. The edges of this concavity may be 
covered with soft vaseline, so that the cavity is rendered air tight when 
the cover glass rests upon it. 
What are impression cover-glass preparations ? 
Ifa clean cover glass is laid upon a given colony and pressed lightly, 
it will (if lifted straight up) have the colony sticking to it and show- 
ing all the peculiarities of arrangement of that colony. 
It may be examined in the same fianner as a hanging drop, or it 
may be dried and stained. In a preparation of this kind, the organ- 
isms are seen in the positions in which they grew. 
EXAMINATION OF THE ATMOSPHERE, SOIL AND 
WATER. 
What is the common method of determining the number and 
variety of bacteria in the atmosphere ? 
A plate of nutrient gelatin is exposed to the air to be examined, for 
a given length of time and then set aside. The organisms that have 
settled upon it from the air go on and develop, but the results are not 
accurate. A modification of this plan is to have a cylindrical glass 
vessel about seven inches long and three inches in diameter. In this 
is set a shallow dish of nutrient gelatin of a diameter of about two 
inches. The vessel is plugged with cotton wool and the whole steri- 
lized. To examine a given atmosphere the cotton wool is removed and 
the dish exposed for a certain number of hours. By this method 
currents of air are avoided and more accurate results obtained. 
Describe Petri’s method of making a bacterial analysis of the air. 
Small plugs or filters of fine sterilized sand, about one inch long and 
one-half inch thick, are made and supported in a glass cylinder. It is 
well to have two such sand-filters in the tube, so that if any bacteria 
pass through the first they will be caught in the second filter. The air 
to be examined is drawn through these filters with an air-pump, which, 
at the same time, measures the amount of air used. The sand is then 
mixed with a nutrient medium and the bacteria allowed to develop. 
The filters and glass tube must be thoroughly sterilized before using. 
In making investigations of this kind with out-door air, what fac- 
tors must be considered ? 
Whether or not the weather is wet or dry (as in wet weather fewer 
bacteria are floating about). Also, the circulation of the air must be 
noted as to whether it is quiet or windy. 
