EXAMINATION OF FILTERS 411 



men under examination is capable of multiplying to the 

 same extent as other specimens of the same organism 

 might multiply in a natural water. In many researches, 

 indeed, in which filters appeared to resist penetration of 

 organisms by growth, it was not even certain whether the 

 organisms under examination could grow in the water at 

 all. The method which must, therefore, be employed is 

 to take water containing known non-pathogenic organisms 

 known to multiply in it at suitable temperatures with 

 sufficient freedom to ultimately penetrate the Pasteur- 

 Ohamberland tube, and to examine specimens of the filter 

 of which the efficiency is to be determined simultaneously 

 and with the same water-supply as specimens of the Pasteur 

 tubes themselYes. The water must be kept at the optimum 

 temperature, and the filtrates examined periodically. If 

 the filter under examination retains the organisms for as 

 long a time as the Pasteur, it must be considered as pos- 

 sessing the same efficiency. If, on the other hand, it 

 passes the test-organisms before the Pasteur tube will 

 do so, it is less efficient, and must for the present be 

 considered insufficient for the prevention of infectious 

 disease. There is an extremely large body of evidence 

 to justify the conclusion that the resistance offered by 

 the Pasteur-Chamberland tube is sufficient to prevent the 

 passage of disease organisms from natural water. This 

 evidence has been collected mainly in all parts of the 

 French possessions, and published by the French Govern- 

 ment: and since the filters have been introduced into this 

 country and India, similar evidence has arisen. There 

 is, however, no evidence to show that the resistance 

 which it offers exceeds that which is necessary for afford- 

 ing trustworthy protection against water-borne disease. 

 It is, therefore, not possible to accept any filter of less 

 efficiency as affording a trustworthy guarantee against 



