408 APPLIED BACTERIOLOGY 



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 cj'lindrical 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 

 fatty globules from milk. The method of its action has 

 not been determined, but it probably depends on some form 

 of surface attraction, as many of the organisms which are 

 arrested are considerably smaller than the pores of the 

 material. It has been shown by repeated experiments that 

 none of the many forms yet tried of biscuit earthenware, 

 having practically the same appearance and analogous 

 composition, possess the same efficiency as the Pasteur- 

 Chamberland material ; but no adequate reason has been 

 discovered for the circumstance. A diagnostic test for the 

 bacterial soundness of the Pasteur-Chamberland tubes is 

 to compress air within them at a pressure of one-half to 

 one atmosphere when the tube has been steeped in water, 

 or is freshly taken from service. If held beneath a body 



