PROCEEDINGS OF THE COTTESWOLD CLUB I23 



first sight appears contradictory to the assertion that the 

 number of bacteria and the amount of organic pollution 

 in a natural water correspond with each other. In puri- 

 fied water, however, the conditions are not the same as in 

 the water of a river where a keen competition is ever in 

 progress between the several species of micro-organisms 

 and where the growth of one is probably preventive of the 

 growth of another. But the fact mentioned above points 

 to the possible dangerous infection of a water of what- 

 ever origin and purity by the organisms of disease, and in 

 fact their chance of multiplying is evidently greatest in a 

 water free from other bacteria. 



When flowing water is brought to rest as in a reservoir, 

 subsidence of suspended matters begins at once to take 

 place, and after standing for some days a considerable 

 improvement is effected in the water, both as to a diminu- 

 tion in the quantity of organic matter shown by chemical 

 analysis, and in the number of bacteria present. The 

 water of streams in times of flood or after heavy rains is 

 less pure than usual, the water becomes turbid with silt 

 washed from the banks and bottom, and this silt contains 

 micro-organisms and organic matter, so that the number 

 of organisms and amount of organic matter is increased 

 in the water at such times. If such turbid water be 

 allowed to stand for a week or ten days the silt subsides 

 and carries down wdth it a very large proportion of the 

 organic matter and the bacteria, and the resulting water 

 is now far purer than the ordinary water of the river, in 

 fact this process of subsidence has almost as great a 

 purifying effect as filtration. 



After standing for several weeks the w^ater begins to 

 deteriorate again, such deterioration being brought about 

 by what may be called the pollution of stagnation. Vege- 

 table and animal growths that belong rather to stagnant 

 than to flowing water begin to develop, multiply, die and 

 decay, and the organic matter, and the micro-organisms in 



