FILTERS 601 



being weighed out and transferred bit by bit with sterilised forceps 

 and scissors to a flask containing 900 c.c. of sterile water. The 

 flask is shaken vigorously, and the washings examined in a manner 

 similar to that employed for shell-fish. 



FILTERS. Reference has already been made to the 

 removal of organisms in water by sand nitration. With 

 regard to niters for domestic use, few of those in the market 

 are capable of doing more than removing particles of 

 suspended matter, while they allow from 5 to 50 per cent., 

 or even more, of the bacteria present in the water to be 

 filtered, to pass through. Such filters are, of course, 

 useless for the prevention of disease in fact, rather favour 

 it, by engendering a false sense of security ; and when in 

 use for some time without cleaning, the water after filtra- 

 tion may be worse, bacteriologically and chemically, than 

 before filtration. 



Woodhead and Wood x found that the only filters which 

 were capable of completely removing organisms were the 

 Pasteur-Chamberland, Berkefeld, and Porcelaine d'Amiant. 

 The Berkefeld, while more rapid in action than the other 

 two, after being in use for a few days may allow some 

 organisms to appear in the filtrate. This, perhaps, is due 

 rather to a growth of organisms through the pores of the 

 filter-candle than to a direct passage. Lunt 2 found that 

 while the ordinary water bacteria, such as the B. fluorescens 

 liquefaciens, appeared in the filtrate from a Berkefeld filter 

 within a few days of the infection of the sample, the typhoid 

 bacillus and the comma bacillus similarly introduced had 

 not passed through the filter four or five weeks after 

 infection. 



Horrocks, 3 however, does not confirm this, and has 

 found that when sterile water is inoculated with typhoid 



1 Brit. Med. Journ., 1894, vol. ii, p. 1053 et seq. 



2 Trans. Brit. Inst. of Prev. Med., vol. i, 1897. 



3 Brit. Med. Journ., 1901, vol. i, p. 1471. 



