i64 PRACTICAL BACTERIOLOGY. 



matter which the distilled water contained must have 

 suflaced for their nourishment and further development. 

 Hence we see that the mere number of the colonies 

 affords us no means of judging of the fitness of the 

 water for drinking purposes ; for, under these condi- 

 tions we should have to condemn distilled water, which 

 has stood for some time, as impure. 



On the other hand, a great deal depends upon the 

 number of hinds present. There can be no increase 

 in the number of species present, however long the 

 water is allowed to stand, so long as it is protected 

 from contamination from without. 



It is further to be remarked that in distilled water 

 very few kinds — one, two, or at most three — are pre- 

 sent, althougli eventually a large number of colonies ap- 

 pear. Also in good, pure spring water from mountains 

 only a very few species are found, and, indeed, in conse- 

 quence of its low temperature, only few individuals at 

 all. On the other hand, if we examine water which 

 has been contaminated by drains, we are surprised at 

 the exceedingly great number of species that we see. 

 Thirty or even more different kinds may be present in 

 only a few drops ; these are easily recognised by the 

 difierent appearance of their colonies. This is readily 

 explained. Impure water has come into contact with 

 many more kinds of bacteria than pure water, and has 

 therefore had much more opportunity of taking them 

 up. If the temperature remains low, these bacteria 



