BAGTEBIOLOGICAL STUDY OF WATER. 587 



In this way one can form an idea as to the approximate 

 number of organisms in the water, and can, in conse- 

 quence, determine the amount of water best suited for 

 the plates. Duplicate plates are always to be made — 

 one set upon agar-agar, which are to be kept in the 

 incubator at body-temperature, and one set upon gelatin, 

 to be kept at from 18° to 20° C. 



As soon as colonies have developed the plates are to 

 be carefully compared and studied. It is to be noted 

 if any difference in the appearance of the organisms on 

 corresponding plates exists, and if so, to what it is due. 

 It is to be particularly noted which plates contain the 

 greater number of colonies, those kept at the higher or 

 those at the lower temperature. In this way the tem- 

 perature best suited for the growth of the majority of 

 these organisms may be determined. As a rule, the 

 greater number of colonies appear upon the gelatin 

 plates kept at 18° to 20° C. ; and from this it would 

 seem that many of the normal water-bacteria do not 

 find the higher temperature so favorable to their de- 

 velopment as do the organisms not naturally present in 

 water, particularly the pathogenic varieties. From these 

 plates the different species are to be isolated in pure 

 culture, the morphological and cultural characteristics 

 determined, and finally, by tests upon animals, it 'is to 

 be decided if any of them possess disease-producing 

 properties. 



Note. — "What use should be made of this observa- 

 tion in examining water for the presence of pathogenic 

 bacteria ? 



The waters most frequently studied from the quali- 

 tative bacteriological standpoint are those suspected 



