102 MICRO-OKGANISMS IN WATER 



stream to a state of comparative bacterial purity. This- 

 case of the Dee is, moreover, specially interesting, as the 

 amount of polluting material gaining access at these 

 several points is so small in comparison with the volume 

 of the stream itself that, as a matter of fact, it was- 

 found impossible by means of chemical analysis to 

 detect any material alteration in the composition of the 

 river, even immediately below each of the above sources 

 of contamination. By means of the bacteriological 

 examination, however, as already indicated, each 

 source of pollution was found to have produced an 

 unmistakable, although transitory, mark on the water of 

 the stream. This repeated restoration of the Dee to a state 

 of comparative bacterial purity is undoubtedly, partially 

 at any rate, attributable to dilution through the increase 

 in volume which takes place during its course. In- 

 deed, in nature it is practically impossible to exclude 

 this complicating factor in the phenomenon of the self- 

 purification of rivers. 



Having obtained some idea of the bacterial contents* 

 of river-water, we will now turn our attention to the 

 examinations which have been made of well-waters, 

 upon which many public and private water-supplies are 

 dependent. 



Well-water. A great many wells have been inves- 

 tigated for micro-organisms, but the data in most cases- 

 are so incomplete that no reliance can be placed upon 

 the numbers found being representative of the well 

 in question. The principal source of error in this- 

 respect is due to the samples in most cases having been 

 collected irrespective of whether the pumps had been 

 in operation or not. Now, the sides of a well are 

 covered with a slimy deposit, and if the water has 

 remained stationary for any length of time ample oppor- 

 tunity is afforded for the extensive multiplication of 

 micro-organisms taking place on such surfaces. It is- 



