BACTERIA IN WATER. 629 
The amount of water which should be added to the usual quan- 
tity of liquefied flesh-peptone-gelatin in a test tube, in order that the 
colonies which develop may be well separated from each other and 
easily counted, can only be determined by experiment. If the water 
is from an impure source a single drop may be too much, and it will 
be necessary to dilute it with distilled water recently sterilized. But 
for ordinary potable water it will usually be best, in a first experi- 
ment, to make two trials, one with one cubic centimetre and one 
with one-half cubic centimetre added to the liquefied nutrient gelatin. 
The water in the collecting bottle should be shaken, to distribute the 
bacteria which may have settled to the bottom, before drawing off by 
means of a sterilized pipette the amount used for the experiment, and 
the germs present in it are to be distributed through the liquefied 
gelatin by gently moving the tube to and fro. 
Koch’s method of preparing a gelatin plate is illustrated in Fig. 
196, A glass dish, containing ice water and covered with a large 
Fig. 196, 
plate of glass, is supported upon a levelling tripod. By means of a 
spirit level this is adjusted to a horizontal position, so that when the 
liquefied gelatin is poured upon the smaller sterilized glass plate, seen 
in the centre of the large plate of-glass, it will not flow, but may be 
evenly distributed over the surface by means of a sterilized glass rod. 
The glass cover resting against the side of the apparatus is placed 
over the gelatin plate while it is cooling, to protect it from atmo- 
spheric germs, and when the gelatin is hard the plate is transferred 
to a shallow glass dish, which is kept ata temperature of about 
20° C. for several days for the development of colonies. Itis difficult 
to count colonies when more than five thousand develop upon a plate 
of the usual size, and for this reason it will be best to repeat the ex- 
periment with a smaller quantity of water from the same source, if 
this is at hand, rather than to attempt to count an overcrowded 
plate. Before pouring the gelatin upon the plate the lip of the test 
tube containing it should be sterilized by passing it through a flame. 
The liquefied gelatin should be carefully distributed to cover a rect: 
