8o 



BACTERIA 



species, those multiplying rapidly and possessing the power 

 of free motility will naturally appear earlier in a filtrate 

 than others. Woodhead and Wood, from their searching 

 and most able investigation, concluded that the Pasteur- 

 Chamberland candle filters (composed of por- 

 celain formed by a mixture of kaolin and other 

 clays) were the only filters out of the substances 

 named above which were reliable and protective 

 against bacteria. They tested over three dozen 

 of the Pasteur filters, and " in every case these 

 gave a sterile filtrate." Pure cholera bacillus 

 in suspension (5000 bacilli to every cc.) and 

 typhoid bacillus in suspension (8000 per cc.) 

 were passed through these filters, and not a 

 single bacillus was detectable in the filtrate. 

 The Berkefeld filter (siliceous earth) came 

 second on the list as an effective filter, and had 

 but the fault of not being a " continuous " 

 steriliser. A certain Parisian filter (" Por- 

 celaine d'Amiante "), made of unglazed porce- 

 lain, rendered water absolutely free from 

 bacteria. Its action was, however, very slow. 



Settin g aside these three efficient filters, we 

 are face to face with the fact that most filters 

 ^o no |- produce germ-free filtrates, even though 

 they are nominally guaranteed to do so. It is professed 

 for animal charcoal, which is widely used, that it absorbs 

 oxygen, and so fully oxidises whatever passes through it. 

 This may be so at first, but after a little use it probably 

 does more harm than good. It appears to add nitrogen 

 and phosphates to water, which are both nutritive sub- 

 stances on which bacteria grow. Moreover it readily ab- 

 sorbs impurities from the air. As a matter of experiment 

 and practice, it has been found by Frankland, Woodhead, 

 and others, that charcoal actually adds to the number of 

 germs after it has been in use for some days. 



|o 



PASTEUR- 



Attached to 

 water Supply 



