78 



BACTERIA 



as protection against disease-producing germs that he advo- 

 cated an adaptation of this plan in places where it was found 

 that a well yielded infected water. Such pollution in a well 

 may be due to various causes ; surface-polluted water oozing 

 into the well is probably the commonest, but decaying 

 animal or vegetable matter might also raise the number of 

 micro-organisms present almost indefinitely. Koch's pro- 

 posal for such a polluted well was to fill it up with gravel to 

 its highest water level, and above that, up to the surface of 

 the ground, with fine sand. Before the well is filled up in 

 this manner it must, of course, be fitted with a pipe passing 

 to the bottom and connected with a pump* This simple 

 procedure of filling up a well with gravel and sand interposes 

 an effectual filter-bed between the subsoil water and any 

 foul surface water percolating downwards. Such an arrange- 

 ment yields as good, if not better, results than an ordinary 

 filter-bed, on account of there being practically no disturb- 

 ance of the bed nor injury done to it by frost. 



The effect of the remedies we have been discussing upon 

 the number of bacteria is demonstrated in the results which 

 Sir Edward Frankland arrived at in his investigation of 

 London waters.* 



Mean of Monthly Examinations for the Year 



* Report on the Metropolitan Water Supply. 



