BACTERIOLOGICAL STUDY OF WATER 611 



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

 difference in the appearance of the organisms on correspond- 

 ing 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 temperature best suited 

 for the growth of the majcH-ity 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 development 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 character- 

 istics 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 observation 

 in examining water for the presence of pathogenic bacteria? 



The waters most frequently studied from the qualitative 

 bacteriological standpoint are those suspected of containing 

 specific pathogenic bacteria — i. e., waters polluted with 

 sewage and with human excreta that are believed to be the 

 source of infection of typhoid fever, or, less frequently, 

 of Asiatic cholera. In the investigations of such water 

 there are several points of which we should never lose sight, 

 viz., unless the water is under continuous study there is 

 only a chance of detecting the specific pathogenic species, 

 for, as a rule, the dangerous pollution occurs either but 



