AIR ANALYSIS. 121 
For the classification of bacteria in milk, see H. 
W. Conn, Report of the Storrs (Connecticut) Agrz- 
cultural Experiment Station for 1899. 
iN 2 eS HAS 
cD . 
Fic. 55.—Sedgwick’s ex- 
panded tube for air exam- 
ination. 
Fic. 54.—Petri’s sand-fil- 
ter for air examination. 
7. Bacteria in the Air.—(a) Prepare 3 gelatin 
plates and expose them to the air for four or five 
