THE FILTRATION OF CULTURES 



71 



others), but the most generally convenient is that in which 

 the fluid is sucked through the porcelain by exhausting the 

 air in the receptacle into which it is to flow. This is con- 

 veniently done by means of a Geissler's water-exhaust pump 

 (Fig. 31, #), which must be fixed to a tap leading directly from 

 the main. The connection with the tap must be effected by 

 means of a piece of thick- walled rubber-tubing as short as 

 possible, wired on to tap and pump, and firmly lashed externally 

 with many turns of strong tape. Before lashing with the tape 

 the tube may be strengthened by fixing round it with rubber 

 solution strips of the rubbered canvas used for mending punctures 



FIG. 33. Chambeiiand's bougie 

 arranged with lamp funnel for 

 filtering a small quantity of 

 fluid. 



FIG. 34. Bougie in- 

 serted through 

 rubber stopper 

 for same purpose 

 as in Fig. 33. 



in the outer cas of a bicycle tyre. A manometer tube (6) and 

 a receptacle (c) (the latter to catch any back flow of water from 

 the pump if the filter accidentally breaks) are intercepted between 

 the filter and the pump. These are usually arranged on a 

 board a, as in Fig. 31. Between the tube /and the pump </, 

 and between the tube d and the filter, it is convenient to insert 

 lengths of flexible lead-tubing connected up at each end with 

 short, stout-walled rubber-tubing. 



Filters are arranged in various ways, (a) An apparatus is 

 arranged as in Fig. 32. The fluid to be filtered is placed in the 

 cylindrical vessel a. Into this a "candle" or "bougie" of 

 porcelain dips. From the upper end of the bougie a glass tube 

 with thick rubber connections, as in Fig. 32, proceeds to flask b 



