METHODS FOR MICROSCOPIC STUDY OF BACTERIA 221 
their walls, to accommodate varying needs.^ Usually the fluid is forced 
through the walls of the filter into the center, which is hollow, by suction. 
The new Seitz filter- (Figs. 18 to 20) possess some advantages over the 
older types. The filtering disc is made of an artificial felt, which has 
uniform porosity. The entire apparatus is readily sterilized, and the 
filtering discs are inexpensive, and very easily replaced. The clear, 
bacteria-free filtrate passes into a sterile container attached to the 
filter. The filters and their necessary accessory parts are sterilized in 
the autoclave for fifteen minutes at 15-pounds live-steam pressure. 
Turbid fluids should be passed through several layers of filter paper 
prior to filtration, to remove the grosser particles which otherwise would 
Fig. 17. — Arnold steam sterilizer. Boston Board of Health type. (Park.) 
soon clog the filter. A time limit, usually not exceeding two hours 
as a maximum, should be set, beyond which filtration should be 
stopped— bacteria may be forced through filters and contaminate the 
filtrate if the process is carried much beyond this interval. IMudd^ 
has shown that motile bacteria may pass through filters where non- 
motile organisms of similar size would usually be held back. The 
physics of bacterial filtration has been shown to be of great significance. 
Mudd^ has studied this problem in great detail and reference should be 
• For description and discussion, see Rosenthal, in Kraus and Uhlenhuth: Handb. 
d. Mikrobiol. Technik, 1924, 3, 1969. 
2 Tron: Centralbl. f. Bakteriol., I Abt., orig., 1924. 93, .380. 
3 Jour. Bacteriol., 1923, 8, 459. 
* See Chapter II, Filters and Filtration, Rivers, 1928. 
