FILTRATION OF BACTERIA 17 



filter-paper. Different porous materials have been used for bacterial 

 filters, of which especially suitable are porcelain, infusorial earth and as- 

 bestos. The filtration apparatus consists of the respective filter and the 

 receptacle which receives the filtrate. Filtration takes place by differ- 

 ences in pressure, where either the fluid is forced through by high pressure 

 or sucked through by a vacuum formed in the receiving vessel. The 

 following are some of the filters most commonly in use. 



1. CHAMBERLAIN'S CYLINDER FILTER, F, used in the Pasteur Institute at Paris. 

 The filter cylinder is made of infusorial earth and may be attached to any water outlet. 



2. PUKAL FILTER, made of burnt kaolin, is used especially for the filtration of large 

 quantities of fluid. The filter b is placed into the beaker e containing the toxin and 

 bacterial fluid. The filter is then closed by a rubber stopper, perforated by a central 

 opening through which runs a glass tube bent at right angles, and this in turn is con- 

 nected with either an air or water pump for producing a vacuum inside of the filter. 

 Between the filter and vacuum pump can be interposed a sterile jar a (Figs. 9 and 10). 



FIG. ii. Reichel filter. 



FIG. 12. Lilliputian filter. 



3. THE REICHEL FILTER (Fig. 1 1) consists of a glass receiver A, having a side neck c 

 and at' the bottom a tube-like outlet d. A porcelain filter B fits into the glass jar and 

 rests upon the margin of the flask by means of a broad collar. The point of junction 

 is made air tight by means of a rubber cap with a central opening, through which the 

 cylinder can be filled. When in use d is shut off by a rubber tube with a pinch cock 

 and c in which lodges a small piece of cotton is connected with a water pump that is 

 instrumental in bringing about a vacuum. The function of d is to allow the removal of 

 samples of the filtrate and finally to obtain the entire filtrate. 



4. THE LILLIPUTIAN FILTER, candle-like in shape, and made of infusorial earth, is 

 employed for the filtration of very small quantities. The filter is cemented upon a 

 metal tube which is screwed, so that it is air tight, into a well-fitting glass cylinder open 

 at the top. The tube is passed through a rubber cork which tightly closes an exhaust 



