PLANTS WHICH CAUSE DECAY 371 



colon bacillus which is characteristic of animal excre- 

 ment, and when found in water shows sewage contami- 

 nation; such water should not be drunk without being 

 boiled, and a sample should be taken to the health 

 officer for examination. 



One fact which cannot help striking us in observing 

 these cultures is the rapidity with which the bacteria 

 grow. A single bacterium produces in forty-eight hours 

 a spot or colony containing thousands on thousands. 

 If we observe the bacteria in a hanging drop (see Fig. 

 165) under the microscope we can see the manner in 

 which they multiply : it consists in simply dividing into 

 two as shown in Fig. 205 (c). It has been observed that 

 this division into two may occur as often as once every 

 half -hour; if this rate could be maintained for twenty- 

 four hours and none should die, we should have at the 

 end of that time more than two hundred and eighty 

 trillions of descendants from a single germ. It may 

 be readily realized that even at a much less rapid rate 

 of increase a few typhoid germs might soon infect a 

 whole water supply. 



In the bacteriological examination of water, it is 

 customary to count the bacteria by the method just 

 described, and also to examine for the colon bacillus 

 and for disease -producing bacteria, such as those of 

 typhoid, cholera, etc. These are identified by their 

 appearance in the cultures and under the microscope. 



It is important to know that typhoid may be con- 



