Filters 87 



that most bacteria are held back jjy filters made of 

 unglazed porcelain. Some species exist, however, that 

 are so small as to pass the minute pores of these filters. 

 For practical purposes, nevertheless, unglazed porcelain 

 may be regarded as bacteria-tight, and water which 

 passes through it as sterile. This fact has led to the 

 use of such filters for the purification of water used in 

 the home. With many people no doubt seems to exist 

 as to their permanent efficiency. Unfortunately, how- 

 ever, there are conditions under which such filters are not 

 bacteria-tight. To be sure, the organisms can not pass 

 through them directly, yet it has been demonstrated 

 quite forcibly that they can grow through them. 



Any of these filters, kept moist for some time, and 

 accumulating organic matter on the inside and in their 

 pores, will, finally, become pervious to the bacteria in 

 them. Typhoid germs, as well as other microorganisms, 

 may thus find their way into the supposedly pure water 

 and cause disease. To render the porcelain filters abso- 

 lutely safe it is necessary to burn them out from time 

 to time, so that all of the organic matter contained in 

 them may be destroyed. Unglazed porcelain filters 

 that can be thus renovated with but slight danger of 

 breaking are now being made. 



Other filters for domestic use, made of charcoal or 

 blocks of sandstone, possess the same defects noted in 

 the porcelain filters, and should not be depended upon 

 for the purification of drinking-water for any consider- 

 able length of time. They should be thoroughly cleaned 

 and boiled at least once a week if their efficiency is to 

 be assured. 



