222 ESSENTIALS OF BACTERIOLOGY 



cylinder closed at one end and pointed at the other. It is 

 placed in another cylinder of glass or rubber, and the pointed 

 portion connected with a bottle containing the water, or directly 

 with the faucet of the water-pipe. The water courses through 

 the porcelain very slowly and comes out entirely free from 

 germs; pipe-clay, bisque, infusorial earth, and kaolin are also 

 perfect niters. The only disadvantage is the long time it takes 

 for the water to pass through. Pressure is used to accelerate 

 the passage in the form of an aspirator or air-pump. 



Fig. 134. Flask fitted with porcelain bougie for filtering large quantities of 



fluid. 



The force of the hydrant water is also sufficient to produce a 

 steady, small stream. 



These porcelain cylinders can easily be sterilized and the 

 pores washed out. 



All the cylinders or bougies are not germ proof, so that they 

 must be tested, and most of them must be cleaned every fourth 

 day, or they will allow germs to pass through. 



Boiling as a Means o) Purifying. When such a filter cannot 

 be obtained, the only alternative is to boil all the water to be 

 used for drinking; and this should especially be done in times 

 of typhoid and cholera epidemics. 



