20 BACTERIA. 



of the rapid oxidation that is going on at the surface, very 

 little free oxygen is left for the use of bacteria at even a 

 short distance from the surface. It will, of course, be 

 objected that this does not apply to the anaerobic bacteria, 

 but in their case it must be remembered that only a certain 

 definite proportion of oxidized material can reach them from 

 above, most of the organic matter having already been 

 converted into inorganic material and used up by growing 

 plants ; the supply is very rapidly cut off, the reduction 

 of what remains after the plants are satisfied being com- 

 pleted, and the bacteria cannot continue to live, because they 

 can no longer obtain any material for their nutrition. 



All the knowledge that has been gained concerning the pro- 

 cess of putrefaction has not been collected in a day, and it was 

 only by a careful marshalling of facts as they were accumulated, 

 and by filling in, sometimes merely with suggestions, gaps that 

 still remained, that the theory of the filtration and biological 

 and chemical purification of water has been built up. But to 

 the practical hygienist it is not sufficient to know that water 

 coming from springs deep down in the earth is free from 

 organisms, and that it may be drawn and consumed with 

 impunity as it rises from the ground. He has something 

 more to consider ; he has to consider under what conditions 

 it can be kept fit for drinking purposes, and by what means 

 it can be most readily and safely distributed to those who 

 use it. He has to remember that water at rest containing 

 organic matter, and exposed to the hot rays of the sun, soon 

 teems with organisms that may or may not be perfectly 

 harmless ; he has to bear in mind the nature of the ground 

 from which water is collected, whether it comes from 

 cultivated areas or from regions in which there is sewage of 

 any kind ; for upon these factors depend the absence or 

 presence of bacteria from the water supply. Further, he 

 may be faced with the problem of how to transform a 

 supply of biologically impure water into a supply fit for 

 drinking purposes. 



The mere chemical analysis of water will not give suffi- 

 cient information to guide a sanitary engineer on these 

 points, though it will indicate to him the lines on which 

 he will have to work. For example, it is quite possible to 

 have a water containing a considerable quantity of organic 

 material which, through its treatment by sand filters, or by 



