EXAMINATION OF FILTEKS 315 



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 

 of water, no air will escape from a sound tube ; but a stream 

 of bubbles will issue from any spot capable of passing bac- 

 teria. This test apparently does not apply to other forms 

 of earthen filters, and for this reason they should not be 

 used for the filtration of serum, or during an epidemic, 

 unless a portion of the filtrate is cultivated, and the bulk 

 retained until it has been proved sterile. This applies 

 particularly to filters made in the Pasteur-Chamberland 

 form, in which a softer material, such as infusorial earth, 

 is used, and a fresh filtering surface is accordingly exposed 

 after each cleaning. Thus, the Berkefeld filter in infusorial 

 earth, of which the tubes may initially be capable of pre- 

 venting the direct passage of organisms, has a small portion 

 of its outer surface removed each time it is cleaned. The 

 consequence is that, sooner or later, a faulty surface is 

 exposed, and the~ tube is liable to pass organisms even 

 before the time when it is worn away sufficiently to break. 

 The Pasteur-Chamberland tubes remain unaffected by 

 cleaning or sterilisation for an indefinite period. They 

 may be sterilised by boiling water, or by saturated steam 



