EXAMINATION OF FILTERS 409 



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 

 under pressure ; or, alternatively, a suitable liquid disin- 

 fectant may be passed through them, with the advantage 

 of dissolving at the same time the whole of the colloid 

 substances deposited in its pores. When used for the fil- 

 tration of blood serum, it is found convenient to allow the 

 mass of blood to drip in ice for forty-eight hours, so as to 

 obtain a serum as free as possible from specks of fibrin ; 

 and the serum given will be readily sterilised by filters at a 

 temperature up to 40° to 50° C. 



The Bacteriological Examination of Water-filters.— The 

 large majority of water-filters at present in use are in- 

 capable of preventing organisms from being washed through 

 into the filtrate. In order to ascertain whether this is the 

 case with any particular filter, it should be sterilised in the 



