122 BACTERIAL ANALYSIS. 
minutes in different places. (In recitation-rooms 
before and after class, out of doors, etc.) 
(4) Cover and allow to grow. 
(c) Examine from day to day, and make cultures 
from the different colonies. 
This is a rough method of determining the rela- 
tive number of bacteria in the air. For more exact 
results recourse must be had to special apparatus 
for aspirating through bouillon or through sugar, as 
described in the text-books. 
8. Bacteria in the Soil.—Numerous species of 
Fic. 56.—Frankel’s instrument for obtaining earth from 
various depths for bacteriologic examination. 
bacteria occur in the soil; some are of special 
interest on account of their pathogenic properties. 
Many are anaérobic, and this fact must be kept in 
mind while studying them. 
To determine the number of bacteria in a sample 
of soil: 
(a) Collect the soil without contamination from 
bacteria from other sources. 
(4) Introduce a measured quantity into a tube of 
liquefied gelatin. Crush with a platinum needle, 
and mix thoroughly with the medium. 
(c) Make plates, count, isolate, and study in the 
usual way. 
