BACTERIOLOGICAL EXAMINATION OF WATER. i6i 



be placed one on the top of the other. In order to 

 make quite sure that no contamination from the air 

 can take place, the dishes may be placed upon a steri- 

 lised glass plate under a sterilised glass bell jar. As 

 a rule, however, this precaution is superfluous, for the 

 impurities arising from contact with the air are gene- 

 rally of a very harmless nature, and usually do not 

 affect the opinion formed of the water. 



In the course of the next few days, the length of 

 time varying according to the temperature and the 

 species present, the colonies make their appearance. 

 They soon become sufficiently large to be seen with 

 the naked eye. Since, however, different kinds grow 

 at very different rates, it can only be assumed about 

 eight days after the appearance of the first colony that 

 all the bacteria have developed, to which, under these 

 circumstances, development is possible. Of course it 

 may happen that all the colonies develop at about the 

 same time, if they all belong to the same species, or 

 to ones that grow at about the same rate. This, how- 

 ever, may be considered as an exceptional case, and it 

 is wise to keep to the given time of eight days before 

 examining the plates to find out the number and kinds 

 of the colonies present. It may, of, course, also occur 

 that even in the plate which was inoculated with only 

 To ccm. of water, so many colonies develop that an 

 examination is absolutely necessary earlier, for other- 

 wise the colonies, which originally were separate, 



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