BACTERIOLOGICAL EXAMINATION OF WATER. 165 



are certain to multiply but little, and thus the number 

 of colonies, even in very impure water, may be less 

 than in pure water, which has been kept for some time 

 at a higher temperature. On the other hand, the 

 number of species in both samples remains unaltered, 

 being affected by neither time nor temperature, for 

 during the interval which is likely to elapse, it is not 

 probable that a species would die off and thus become 

 extinct in the sample. Thus we see that the number 

 of species furnishes a far more reliable criterion for 

 forming an opinion of the water than is afforded by 

 the number of colonies. 



Finally, the character of the species found is of great 

 importance. If the water is to be examined, in order 

 to find out whether it is fit for drinking purposes, we 

 have first to notice if there are many species present 

 which liquefy the gelatine. These kinds are almost 

 invariably putrefaction bacteria, in the exact sense of 

 the word. They only thrive well where plenty of 

 organic material suitable for their nourishment is 

 present, being almost entirely absent in pure water. 

 Thus they are exacting in their requireoients, and are 

 unable to multiply in pure water even at a high tem- 

 perature. This may be proved by inoculating pure 

 water with a minute quantity of such a colony; after 

 careful mixing, some of this is poured off immediately 

 into a culture dish, and after eight days some more 

 into another. We then see that, whilst those bacteria 



