254 BUTTER-MAKING. 
out the coarse organic and inorganic matter held in suspension, 
without removing the bacteria. If some bacteria are removed 
with the matter held in suspension, others are carried along 
from the filter-bed. Owing to this, a new filter-bed must be 
kept in operation a few days before the filtered water can be 
considered pure and ready for use. The following table illus- 
trates how the germ-content of water is decreased as the process 
of filtration is carried on during the first few days: 
Filtered Unfiltered 
Water, Tap-water, 
Germs per Germs per 
Cc. c. cc. 
No. 1. Taken when filter-bed was first used......... 20,000 107 
Oe Dan ne ae ne had worked 1 day..... 860 118 
Lae: yam ss us ss «« 3 days..... 370 96 
oe 4, ae “e oe ce “cc 5 iZ3 ey aed 48 54 
ce 5. cé ce “ce 6é ce 7. ce ous 3 73 
oe 6. fag oe ce “ec ce 9 “e Hoe hs 5 89 
It will be seen from the table that during the first three 
days the filter-bed was in use the filtered water contained more 
germs than the unfiltered. Good results were not obtained 
until the seventh day. In order to be on the safe side it is 
best to expose the filter-bed to continuous filtration for about 
nine days before the water is used. 
The slimy coat referred to above is formed by certain germs. 
These germs then constitute the real agent of filtration. In 
order for these micro-organisms to do efficient work oxygen 
is essential. Well-water of average purity contains enough 
oxygen in solution without employing an intermittent process of 
filtration, and consequently for creamery purposes the con- 
tinuous method of filtration is to be recommended. 
Intermittent.—The intermittent process of filtration is used 
where comparatively impure water is being purified, such as 
in purifying water for large cities. If the continuous process 
of filtration were employed in such instances, the filtered 
water would not be free from germs, due to the fact that impure 
river-water does not carry enough oxygen in solution to supply 
the germs which form the real filtering agency. 
