FILTRATION. 45 
glazed porcelain. The most satisfactory filters in 
use at present are the Chamberland and the Berke- 
feld. 
Fic. 20.—Filter: a, 
porcelain bougie; 4, at- 
tachment for  suction- 
pump; ¢, reservoir; d, 
sterile receiver ; ¢, rubber 
tube wired to bougie and 
reservoir. 
Fic. 21.—Filter: a, 
porcelain bougie; 4, at- 
tachment for suction- 
pump; ¢, reservoir; d, 
sterile receiver; ¢, perfo- 
rated rubber collar; 7 
glass chimney for draw- 
ing the fluid around the 
bougie (Page). 
Bacteria in the air are unable to pass through a 
cotton-wool filter, and consequently sterile flasks or 
