BACTERIOLOGIC EXAMINATION OF WATER. 173 



the filter, the benefit is not permanent. The filters must 

 frequently be renewed. Porcelain filters seem to be the 

 only positive safeguard, and even these, the best of which 

 seems to be the Pasteur-Chamberland, allow the bacteria 

 to pass through if used too long without renewal or with- 

 out firing. 



