BACTERIOLOGICAL EXAMINATION OF WATER. 151 



it, and thus, especially if there are only a few of them 

 present, it may easily happen that there are none in 

 the few cubic centimetres of water used for the plate 

 cijltivations, whilst at the same time there may be 

 sufficient in other portions to cause infection. The 

 most important fact however is this : that between the 

 time of infection and the outbreak of the disease a 

 considerable period of time frequently elapses, so that 

 it is possible that although a great many bacteria were 

 present in the water at the time of infection they may 

 have disappeared a long time before the examination 

 of the water takes place. This is easily to be under- 

 stood when the water passes along pipes, for it soon 

 flows away, and perhaps only an hour after the en- 

 trance of the bacteria they may have been all carried 

 away. In wells the case is somewhat different. If 

 the bacteria do not find suitable conditions, they sink 

 to the bottom and die. If therefore the sample is 

 taken from the upper water of the well, it may be 

 quite free from bacteria, whilst there may be living 

 specimens still present in the lower portions of the 

 water. 



We must therefore once for all give up in such 

 examinations the hope of demonstrating the typhoid 

 bacillus; and must regard its discovery as a special 

 piece of good fortune. Nevertheless in these cases 

 the bacteriological examination of the water affords 

 us certain indications on which we must base the 



