BACTERIOLOGICAL STUDY OF WATER 



617 



whole apparatus (depicted in Fig. 106) is provided with a 

 weight that insures its sinking, and a heavy cord by which 

 it may be lowered and raised. It should be sterilized before 

 using. After collecting the sample the 

 bottle should be wiped dry with a ^''^- ^"^ 

 sterilized towel. Before removing the 

 stopper the mouth of the bottle should be 

 rinsed with alcohol 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 analy- 

 sis of water with which one is not ac- 

 quainted certain preliminary steps are 

 essential. 



It is necessary to know approximately 

 the number of organisms contained in 

 any fixed volume, so as to determine 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 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 





A 



Bottle for collecting 



water. 



