CHAPTER XIII. 



QUALITATIVE ANALYSIS continued. 



CLASS III. 



THE most important of all the micro-organisms which appear 

 in water are those which produce specific disease in human 

 beings. It is not absolutely proved that the causes of most 

 infectious diseases may not appear in water ; but at the present 

 time it is exceedingly difficult to show the presence of any of 

 them except the spirillum of cholera, and perhaps the 

 B. typhosus. 



Bacillus Typhosus. 



This micro - organism was first observed by E berth, and 

 more fully investigated by Gaffky. Its characteristics are as 

 follows : 



Gelatine Plates. The surface colonies appear as thin bluish 

 films with an irregular margin. Under a low power markings 

 are seen, which look like ridges and valleys, running irregu- 

 larly from the centre to the periphery ; sometimes the mark- 

 ings are very faint, and the colony resembles a very thin film of 

 glass. The colonies in the depth of the gelatine are round or 

 oval in shape, more or less translucent, and appear filled with 

 highly refracting granules. As a rule, the colonies in gelatine, 

 when incubated at 20 to 22 C., grow very slowly, requiring 

 seventy-two to ninety-six hours incubation before they show 

 their characteristic appearances. The gelatine is not liquefied. 

 Agtir Plates. - - The surface colonies are white, semi- 

 opaque, with a circular or slightly irregular margin. Under 

 a low power surface markings of ridges and valleys are seen, 

 but they are not usually so distinct as in the gelatine plates. 

 Sometimes the centre appears thicker than the periphery, 

 giving the colony a somewhat ringed appearance. When 



