AEROSCOPES 265 
length and 6 mm. in diameter bent at an angle of 45° in order to prevent precip- 
itation of bacteria or dust particles into the aeroscope. In using this aeroscope 
a measured volume of air is filtered through the tube, the sand shaken out into 
10 ¢.c. of water and aliquot portions of this suspension plated on nutrient agar. 
The “ modified’? form of the aeroscope differs very little. The 
lower rubber stopper and the bolting cloth is eliminated by fusing a 
small tube into the large one. A layer of cotton supports the sand. 
SES 
== 
ea 
Standard Modified Standard Rettger 
Aeroscope Aeroscope Aeroscope 
Fig. 57.—Types of Aeroscopes. (After Ruehle.) 
In order to render the apparatus capable of being sterilized by hot air, 
the upper stopper is made of cork. 
The Committee on Standard Methods for the Examination of Air 
of the American Public Health Association recommended the following 
procedure: 
The number of bacteria in the air does not appear to be a factor of any great 
significance. In special cases, however, as in the study of dairy conditions, it 
may be of interest. 
The sand filter method originally used by Petri has proved essentially sound 
