178 BACTERIOLOGY. 



in the water, and can in consequence determine the 

 amount of water necessary to use for each set of plates. 



Duplicate plates are always to be made — one set upon 

 agar-agar, which are to be kept at the body -temperature, 

 and one set upon gelatin, which are to be kept at a tem- 

 perature of 18° to 20° C. 



As soon as the colonies have developed, the plates are 

 to be carefully compared and studied. It is to be noted 

 if any difference in the appearance and number of 

 organisms on corresponding plates exist, and if so on 

 which plates the larger number of colonies have de- 

 veloped. In this way the temperature most favorable 

 for the growth of most of these organisms may be 

 determined. The opinion has been advanced that many 

 of the organisms constantly present in water, which 

 make up its normal flora, develop better at a lower than 

 at a higher temperature. This will not be the case, 

 however, if pathogenic forms are present, because they, 

 as a rule, require the body^temperature for their most 

 favorable development, though some of them do grow 

 very well at a lower temperature. 



The isolation of the different species and their sys- 

 tematic study is to be conducted in the way given for all 

 bacteria. 



The Quantitative Estimation of Bacteria in 

 Water. — The quantitative analysis requires more care in 

 the measurement of the exact volume of water employed, 

 for the results are to be expressed in terms of the number 

 of individual orgauisms 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 the multiplication of the organisms 

 during transit is so great that the results of analyses 



