80 OUTLINES OF BACTEEIOLOGY 



of the soil in larger quantities than during the remainder of the year, 

 and, as will be seen presently, small quantities of soil usually contain 

 many thousands of bacteria. When the water of a river flowing through 

 a town becomes contaminated, as generally happens, especially if the 

 sewage of the town be allowed to flow into the river, the bacterial con- 

 tent of the water becomes enormously greater. The following table 

 shows how the Severn is affected by its passage through Shrewsbury : 



BACTERIA PER c.c. 



Two miles above Shrewsbury, 7,000 



Waterworks opposite Shrewsbury, 13,000 



Ferry L, 0*6 miles lower down, 20,000 



English Bridge, 1-6 miles lower down, - 23,000 



Ferry III., 2'5 miles lower down, - 19,000 



Uffington, 4*7 miles lower down, - 17,000 



Alcharn, 9 miles lower down, 13,000 



Cressage, 16 miles lower down, - 5,000 



This table also shows that rivers can purify themselves, for we see that, 

 a few miles below Shrewsbury, the Severn has returned to its normal 

 conditions. It is important to understand the means at the disposal of 

 rivers whereby purification is effected. The four chief agencies are 

 oxidation, dilution, deposition, and competition among the different 

 organisms. The first cuts off the food supply, and bacteria cannot 

 multiply unless the necessary organic pabulum is present. When this 

 becomes oxidised, it is rendered unfit for consumption by bacteria, 

 which consequently die of starvation. In the case of rivers, the water is 

 oxygenated by absorption at the surface, this process being facilitated 

 by the movement of the water, whilst weirs and waterfalls cause 

 a considerable increase in the amount of oxygen in the water. The 

 effect of the second agency is seen when it is considered that the 

 sewage and other liquid pollutions come into contact with a greater 

 volume of liquid as they move down with the river, so that the 

 number of organisms per unit volume becomes less. The third agency, 

 viz. deposition, is a very important factor, for, as the sewage is carried 

 down, all the participate matter, sooner or later, sinks to the bottom. 

 It might appear that the bed of the river would thus become, in course 

 of time, an obnoxious breeding place for bacteria, but such is not the 

 case. The bacteria have to enter into competition with other organisms 

 which require the same organic pabulum. In this way water plants, 

 especially the lowest forms, the algae, and the minute animals that are 

 found in the water, make great inroads on the organic pabulum, with 

 the result that the food supply of the bacteria is cut off. In addition, 



