124 THE STORY OF THE BACTERIA. 



diluted. The sewage may contain one hun- 

 dred thousand typhoid germs to one teacupful, 

 while the diluted mixture has in it not more 

 than one to the same volume. But it should 

 never be forgotten that the one germ is capa- 

 ble of multiplying in the human body to an 

 enormous extent, and for this reason, in the 

 living bacterial poison dilution is of much less 

 significance than in ordinary poisons which are 

 not alive and self-propagating. 



The fact is, that in view of all that we have 

 seen of the nature of bacteria and their dis- 

 ease-producing powers, sewage-polluted water 

 from wells, or springs, or rivers, or lakes, ought 

 not to be used for drinking and culinary pur- 

 poses without some system of purification 

 which is demonstrably efficient. 



The new methods of bacterial analysis of 

 water, which have been described in the earlier 

 pages of this book, now enable us to tell with 

 a great deal of certainty, sometimes with and 

 sometimes without a chemical analysis, whether 

 or not a given water has actually been polluted 

 with sewage, or human or animal waste, and 

 whether the modes of purification to which it 



