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. 
