314 APPLIED BACTERIOLOGY 



No general rule can be given for the depth to which the 

 top layer must be removed, as it varies with the nature of 

 the water and sand, temperature, etc. 



The ordinary rules for the selection of the epochs for 

 starting and arresting the filters, and the operation of 

 removing the upper surface, require considerable experience 

 and judgment; and it frequently happens that through 

 carelessness or unavoidable mistake the filtration is im- 

 perfect. Thus, in 1894 the filters at Nietleben, Altona, 

 Hamburg, and Stettin, being over 10 per cent, of the total 

 sand-filters in use in Germany, where great attention has 

 been given to the subject, passed the cholera organism, 

 and permitted epidemics in their towns. In the same year 

 the typhoid organism, of which the detection was difficult 

 and uncertain with the means then at disposal, was, never- 

 theless, found beyond doubt in the Berlin water-mains. 



Bacterial Filters. — It is obvious that for the purpose of 

 bacteriological investigation such appliances as have been 

 described are practically useless. Pasteur and Chamber- 

 land investigated a large number of earthen materials, 

 beginning with ordinary biscuit porcelain. They found 

 them to present very different degrees of resistance to the 

 passage of bacteria. The difference did not appear to 

 correspond to either the density of the material or the rate 

 of filtration, in many cases a material of closer grain and 

 less rapid output giving worse results than other materials 

 more open in structure and more rapid in filtration. They 

 ultimately found that the best results were obtained with 

 a particular mixture of earths prepared with a special 

 manipulation ; and it is these substances which, when 

 made in the well-known cylindrical form, constitute the 

 Pasteur-Chamberland filter. This filter is found to be 

 perfectly trustworthy in the removal of all organisms from 

 liquids ; it also retains any particulate matter, such as the 



