288 MICRO-ORGANISMS IN WATER 



This method, reasonable enough as it may appear, is 

 not without certain objections, for in the first place the 

 quantity of water which can be thus examined is rela- 

 tively very small, and it is highly probable, therefore,, 

 that if only a few individual microbes were present in 

 the water they might very easily escape detection alto- 

 gether ; whilst, secondly, the bacteria may, during 

 their residence in the water, have their vitality so far 

 enfeebled that they are unable to grow on a gelatine- 

 plate, which often proves a somewhat adverse medium 

 for the development of micro-organisms in a weakly 

 condition. 



Straus and Dubarry, and Percy Frankland in his 

 later experiments on anthrax in water, have made a 

 practice in particular cases of finally adding nutritive 

 broth to the various samples of water. In this mannery 

 if there are only a few microbes still remaining alive 

 in the water, they will, on the addition of the broth^ 

 undergo abundant multiplication, and their presence can 

 then be easily revealed by subsequent plate-cultures. 

 Although in this manner we lose the means of esti- 

 mating the actual numbers in which the particular 

 pathogenic bacteria under observation are present in 

 the water, we acquire more exact information as to the 

 real duration of their vitality. 



In addition to the question of vitality, it is of course 

 also of the highest importance to ascertain whether the 

 bacteria have retained their virulence or not, but this 

 can only be done in the case of those bacteria which 

 produce powerfully pathogenic effects on animals. 

 Eeference to the experiments which have been carried 

 out in this direction will also be found embodied in the 

 following tabular records of these investigations on the 

 vitality of pathogenic bacteria in water. 



