256 BACTERIOLOGY. 



almost absolutely free from bacteria, and if they are 

 frequently cleansed they are reliable. A large Berke- 

 feld filter will allow sixty gallons of water to pass per 

 hour. The Pasteur filter is more compact and slower. 

 From the best Pasteur filters sterile water may be 

 passed for two to three weeks; from the Berkefeld 

 usually only a few days. A simple typical low- 

 pressure filter is that of Bailey Denton. The upper 

 compartment contains the filtering material, which 

 may be sand or charcoal, and is fed from a cistern or 

 hydrant. After a certain quantity of water has passed 

 in the supply is automatically cut off until the whole 

 amount has filtered. A filter easily made is the follow- 

 ing : Take a large-sized earthenware pot and plug the 

 hole in the bottom with a cork, through which pass a 

 short glass tube. Upon the bottom place an inch of 

 small pieces of broken flower-pot; upon this a couple 

 of inches of well-washed small gravel, and upon this 

 six to twelve inches of well- washed fine, sharp sand. 

 Cover the sand with a piece of filter-paper and hold 

 this down with a few small stones. Mount the pot on 

 a tripod, and it is ready for use. The paper prevents 

 the sand being disturbed when water is added, and as 

 it also holds most of the sediment, this can be readily 

 removed. Every few months the sand can be washed 

 and replaced. Animal charcoal is not a good substance 

 for permanent filters, as bacteria grow well in it. 

 Whenever water is suspected, and there is any doubt 

 as to the filters, it should be boiled for ten minutes; 

 this will destroy all bacteria. This precaution should 

 always be taken in the presence of typhoid fever and 

 cholera epidemics, 



