BACTERIOLOGICAL STUDY OF WATER. 629 



course must then be had to all the differential tests 

 detailed in the chapter on that organism. 



THE QUANTITATIVE ESTIMATION OF BACTERIA IN 

 WATER. Quantitative analysis requires more care in 

 the measurement of the exact volume of water em- 

 ployed, for the results are to be expressed in terms of the 

 number of individual organisms to a definite volume. 

 The necessity for making the plates at the place at 

 which the sample is collected is to be particularly 

 accentuated in this analysis, for multiplication of the 

 organisms during transit is so great that the results 

 of analyses made after the water has been in a vessel 

 for a day or two are often very different from those that 

 would have been obtained on the spot. 



NOTE. Inoculate a tube containing about ten cubic 

 centimetres of sterilized distilled or tap water with a 

 very small quantity of a solid culture of some one of 

 the organisms with which you have been working, 

 taking care that none of the culture-medium is intro- 

 duced into the water-tube and that the bacteria are 

 evenly distributed through it. Make plates at once 

 from this tube, and on each succeeding day determine 

 by counts whether there is an increase or diminu- 

 tion in the number of organisms *. e., if they are 

 growing or dying. Represent the results graphically, 

 and it will be noticed that in many cases there is 

 during the first three or four days a multiplication, 

 after which there is a rapid diminution ; and, if the 

 organism does not form spores, usually death in from 

 ten to twelve days. This is not true for all organisms, 

 but does hold for many. 



