180 BACTERIOLOGY. 



from which to prepare plates or Esmarch tubes on the 

 spot, or the tip of the stem may be re-sealed in the 

 flame of an alcohol lamp, the bulb packed in ice, and 

 transported in this condition to the laboratory. 



In beginning the quantitative analysis of water with 

 which one is not acquainted, there are certain preliminary 

 steps that are essential. 



It is necessary to know approximately the number of 

 organisms contained in any fixed volume, so as to de- 

 termine the quantity of water to be employed for the 

 plates or tubes. This is done usually by making pre- 

 liminary plates from one drop, two drops, 0.25 c.c, 

 0.5 c.c, and 1 c.c. of the water. After each plate has 

 been labelled with the amount of water used in making 

 it, it is placed aside for development. When this has 

 occurred, one selects the plate upon which the colonies 

 are only moderate in number — about 200 to 300 colonies 

 presenting — and employs in the subsequent analysis the 

 same amount of water that was used in making this 

 plate. 



If the original water contained so many organisms that 

 there developed on a plate or tube made with one drop too 

 many colonies to be easily counted, then the sample must 

 be diluted with one, two, or three volumes, as the case 

 may be, of sterilized distilled water. This dilution must 

 be accurate, and its exact extent noted, so that subse- 

 quently the number of organisms per volume in the 

 original water may be calculated. 



The use of a drop is not sufficiently accurate. The 

 dilution should therefore always be to a degree that will 

 admit of the employment of a volume of water that may 

 be exactly measured, 0.25, 0.5 c.c. being the amounts 

 most convenient for use. 



