628 BACTERIA IN WATER. 
nearly filled with water that it is not so simple a matter to obtain the 
contents for our culture experiments without undue exposure to at- 
mospheric germs. In practice small glass bottles with ground-glass 
stoppers will be found most convenient, and, when properly steril- 
ized, are unobjectionable. They should be filled at a little distance 
below the surface, as there is often a deposit of dust upon the surface 
- of standing water, and sometimes a 
: delicate film made up of aérobic bac- 
‘ teria. When water is to be obtained 
: from a pump or a hydrant it should 
x be allowed to flow for some time before 
the collection is made. To collect 
water at various depths the apparatus 
shown in Fig. 195 isrecommended by 
Lepsius. An iron frame supports an 
inverted flask, A, filled with sterilized 
mercury and containing about three 
hundred cubic centimetres. The flask 
B is intended to receive the mercury 
when, at the desired depth, it is al- 
c al lowed to flow through the capillary 
tube 6. Thisis sealed at the extremity 
i) and bent as shown in the figure. By 
ON pulling upon the cord ¢ this tube is 
an broken, and as the mercury flows from 
the flask this is filled with water 
through the tube a. The extremity 
of the broken tube b is closed by the 
mercury in the flask B when A is full 
of water, and the apparatus can be 
brought to the surface with only such 
water as was collected at the depth 
from which a sample was desired. 
The bacteriological analysis is 
made by adding a definite quantity 
of the water under investigation to 
liquefied gelatin or agar-gelatin, and 
making a plate or Esmarch roll tube, which is put aside for the devel- 
opment of colonies. Miquel and others have preferred to use liquid 
cultures and the method of fractional cultivation described in the 
previous section, The use of a solid culture medium has, however, 
such obvious advantages that we do not consider it necessary to do 
more than refer to the other method as one which, when applied 
with skill and patience, may give sufficiently accurate results. 
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