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Effect of Filtration. 
At first sight the results of the filtration are most disappointing as 
nstead of t e reduction of 90 per cent, in the number of organ.sms obtained by 
toation in English and Continental filter beds the number of organisms 
n the filtrate was actually greater on some occasions than that m the irate 
"^O.fcbsefexamination, however, it is seen that when most required and 
die number of organisms in the reservoir is greatest an enormous reduction 
k m ode in the number of organisms. The use of the filter beds pre\ ents 
the influx of the washings of the streams and render the quality of the water 
Un 'T ut! her an"exarmnation of the character of organisms in the filtered 
water shows that nearly all are spore bearing non-pathogenic organisms and 
that the results for some weeks were 
nuite satisfactory as, excluding the examination made after the filter had 
been in use for i 5 hours which of course was too early for bacterial filtration 
t nt ak enlace it was not till the bed had been in use 28 days that the acid 
lUtstnning orgmusmsrvere numerous and even 8 days later they were 
r6dU Indol forming ^‘ganfsmfwere not present nor were they at other 
examinations made of the filtered water i month, 7 weeks, and 116 days 
respectively after the use of the filter bed had been commenced __ 
V The benefical effects of filtration are seen from the table to be . 
(1) The removal for a prolonged period of indol forming 
organisms. . 
i 2 ) The removal for a shorter period of a large proportion of the 
organisms which form acid and gas in MacConkey s medium. 
These two classes are those most likely to be dangerous to man. 
(,) Equalisation in the number of organisms present and 
protection of the town supply from the first washings of the ground 
after heavy rain. 
These effects are, in my opinion, quite sufficient to show the necessity 
° f fil The°mganisms which at first pass through the filter are all spore 
bearing organisms usually very similar to B. subti is. nisom ^ 
determinations 80 to 90 per cent, of the organisms present in the filtrate 
W6re in f ‘the’examinations made after 3 weeks the other organisms were 
present in constantly increasing numbers so that under the conditions o 
work of these filters they should only be used for a comparatively sho 
period, not more than a month to get goo res H..y , r much 
P In Europe and in temperate climates a filter can be run for much 
longer, but no doubt the higher temperature and gr.eater vainety of 
organisms are factors in the more rapid growth through the filter 
organisms 0^01, if possible, to prolong the period of use of a filter bed 
and also to obtain greater efficiency. It will be noted that filtration ha. 
