BACTERIA IX WATER. 639 



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. 

 198. A glass dish, containing ice water and covered with a large 



FIG. 198. 



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 at a temperature of about 

 20 C. for several days for the development of colonies. It is 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- 



