i68 PRACTICAL BACTERIOLOGY. 



put at 500. This is called the "fixed point." As 

 we have seen, the number of colonies furnishes the 

 least reliable of the data by which we have to judge, 

 being the one most affected by outside circumstances, 

 and therefore of the least value. 



Thus the opinion formed of the water must depend, 

 within certain fixed limits, upon individual judgment, 

 and it is only after long practice, and after having 

 compared many different kinds of waters, that the 

 student is in the position to form a correct opinion. 



The method of examination is somewhat different 

 if the water is to be examined for pathogenic organ- 

 isms. In by far the greatest number of cases the 

 typhoid bacillus is the one searched for, and on this 

 account, in addition to the ordinary examination as 

 given above, those colonies of bacilli which resemble 

 those of the typhoid bacillus are further investigated. 

 There are quite a large number of bacilli, whose 

 colonies in gelatine plates are very similar to those of 

 the typhoid bacillus. A small portion of each suspected 

 colony is taken with a sterilised needle, and is spread 

 out in a drop of water upon a slide. If with the 

 microscope the colonies still resemble those of the 

 typhoid bacillus, portions must be inoculated on to 

 carefully sterilised cooked potatoes, and into tubes of 

 gelatine. It is best then to compare these cultures 

 with similar ones prepared with typhoid bacilli; 

 especial notice must be taken of the potato cultiva- 



