CHAPTEE XXVII. 



Bacteriological study of water Methods employed Precautions to be 

 observed Apparatus used, and methods of using them Methods of investi- 

 gating air and soil. 



THE conditions that favor the epidemic outbreak of 

 typhoid fever, Asiatic cholera, and other maladies of 

 which these may be taken as types, have served as a 

 subject for discussion by sanitarians for a long time. 



Of the hypotheses that have been advanced in ex- 

 planation of the existence and dissemination of these 

 diseases, two stand pre-eminent and are worthy of con- 

 sideration. They are the " ground- water' 7 theory of 

 von Pettenkofer and his pupils, and the " drinking- 

 water 77 theory of the school of bacteriologists of which 

 Koch stands at the head. 



The adherents to the " ground- water 77 view explain 

 the presence of these diseases in epidemic form through 

 alterations in the soil resulting from fluctuations in the 

 level of the soil-water, and assign to the drinking-water 

 either a very insignificant role, or, as is most frequently 

 the case, ignore it entirely. On the other hand, those 

 who have been instrumental in developing the drinking- 

 water hypothesis claim that alterations in the soil play 

 little or no part in favoring the appearance of these dis- 

 eases in a neighborhood ; but that, as a rule, they appear 

 as a result of direct infection, through the use of waters 

 that are contaminated with materials containing the 

 specific organisms that are known to cause such diseases. 



