122 PROCEEDINGS OF THE COTTESWOLD CLUB 



the water is saturated is rapidly reduced. In stagnant 

 water it takes a comparatively long time for fresh aeration 

 to take place, and the process of destruction is corres- 

 pondingly slow, but in running water fresh oxygen is 

 rapidly absorbed, and the process is as quickly completed. 

 It has long been known that streams, however polluted, 

 are capable of recovering their purity after a flow of a 

 few miles, and originally the fact was attributed to the 

 simple eff"ect of oxygen, but it is now universally recog- 

 nised that the process is effected through the mediation 

 of bacteria. It is a vital process, and is precisely analagous, 

 or one might say identical to what is constantly going on 

 in soil. In soil, as in water, complex organic material is 

 being constantly reduced by bacterial action to simple 

 salts and gases, the ubiquity of micro-organisms preventing 

 the permanence of organic filth. An example might be 

 given in the contents of a dry-earth closet, which in an 

 incredibly short time is reduced to an absolutely homo- 

 geneous soil, differing only from the original earth in 

 containing a slight addition to its soluble salts. The 

 action of micro-organisms upon organic matter in water 

 is similar, but proceeds at a faster rate. The micro- 

 organisms are not permanently increased in a river by its 

 temporary pollution, although in a general way their 

 numbers accord with the amount of pollution present in 

 any particular portion of the river. In short their numbers 

 are regulated by the amount of material that is presented 

 to them for destruction, and in examining an untreated 

 river water both chemically and bacterially the same infor- 

 mation is obtained, for one is quite sure by experience 

 that when the water is less pure than usual there is likely to 

 be found a greater number of bacteria contained in it. When 

 water has been purified, however, by filtration, or by boil- 

 ing or distillation, and thus wholly freed from the bacteria 

 that it previously contained, fresh bacteria being introduced 

 will now grow in it with great rapidity, a fact which at 



