172 MICRO-OEGANISMS IN WATER 



through these various filters, as showing again how 

 insignificant may be the chemical action of materials 

 causing a marked biological improvement ; the samples 

 were collected when the filters were working with their 

 greatest efficiency. 



Domestic Filters 



Pasteur-Chamherland Filte7\ — In this filter the 

 water is made to pass through a cylinder of biscuit- 

 porcelain, the number of such cylinders depending of 

 course upon the amount of filtered water required. The 

 one experimented upon by one of us ^ consisted of ten 

 such cylinders. Ordinary filtered Thames water was 

 forced through under a pressure of 30 to 40 feet of water. 

 Under these circumstances the filter commenced by 

 yielding 1 litre in 40 minutes, or 36 litres per 24 hours ; 

 but already, at the end of a fortnight's continuous 

 action, it was only delivering 1 litre in 1 hour 14 

 minutes, or rather less than 20 litres per 24 hours, and 

 after 2-J months the rate of filtration was 1 litre in 1 

 hour 22 minutes, or 17^ litres in 24 hours. 



Chemically again this filter was found to have but 

 a very trifling influence upon the composition of the 

 water, the only change being a slight diminution in the 

 amount of mineral matter present. Biologically, on 

 the other hand, it was, at any rate when new, very 

 efficient, the complete removal of the micro-organisms 

 being accomplished. Numerous and elaborate investi- 

 gations as to the efficacy of this filter have since been 

 made by various bacteriologists, with the result that, 

 although in the first instance it is fonnd to yield germ- 

 free water, it loses this power after being in action a 



^ 'Eemoval of Micro-organisms from Water.' Percy Fraiikland, Boy, 

 Soc. Proc, 1885. 



