596 BAOTEBIOLOOY. 



and heated with a gas-flame, to prevent contamination 

 of its contents by matters that may have been upon 

 its surface. 



In beginning the quantitative analysis of water with 

 which one is not acquainted certain preliminary steps 

 are essential. 



It is necessary to know approximately the number of 

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

 mine the quantity of water to be employed for the plates 

 or tubes. This is usually done by making preliminary 

 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 mod- 

 erate 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, ten, twenty-five, fifty, 

 or one hundred volumes, as the case may require, of 

 sterilized distilled water. This dilution must be accM- 

 rate, and its exact extent noted, so that subsequently 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 and 0.5 c.c. being the 

 amounts most convenient for use. 



Duplicate plates should always be made, and the 



