MEANS OF DEFENCE AGAINST MICROBES. 249 



in the intestines is always due to the use of unfiltered 

 water, and this should enforce the general use of 

 filters, which is often neglected even by those who 

 cannot be deterred by the relatively moderate cost 

 of an instrument which it is almost impossible to 

 wear out. An ordinary filter, however, can arrest a 

 very small proportion of microbes, which are much 

 more minute than the ova of ascarides. 



A filter has, therefore, been devised, so perfect as 

 to allow the passage of no solid matter in suspension, 

 not even the most minute organisms contained in 

 drinking-water. This result is effected by the filter 

 invented by Chamberland in Pasteur's laboratory. 

 The filter is formed (Fig. 101) of a vessel of biscuit- 

 ware, A, shaped like a candle (whence its name of 

 bougie Chamberland) ; this is fastened to the lower 

 part of the metallic receiver D, which receives under 

 pressure the water coming from the cock E. This 

 vessel consequently filters the water from without to 

 within, and it flows through the orifice B, perfectly 

 free from solid particles, as it appears from a micro- 

 graphic examination. 



Fitted to the distributing water-taps of many 

 houses in Paris, and especially in lycees, the Cham- 

 berland filter acts under the normal pressure of the 

 water-conduit, and, by a new modification of the 

 inventor, can even act without such pressure. For 

 this purpose he arranges his filters in a battery, from 

 eight to ten or more, in a cylindrical receiver, closed 



