SEWAGE OUTFALLS AND DISCHARGE 319 



deposit to a great distance down, and to shift it from place to 

 place. 



(b) Although on emerging from the sewers the solids are for 

 the most part well broken down, yet a very considerable amount 

 of the lighter faeces, various debris, and pieces of disintegrated 

 paper float for a long distance down the stream, and disfigure 

 the banks. In the little bays where these accumulate, the mud 

 is blacker than elsewhere, and the willows have strained out a 

 considerable quantity of the floating material ; offensive decom- 

 positions occur ; the water usually presents a scum ; the bacteria 

 are more numerous and B. coli more abundant. The course 

 taken by floating matters was traced by throwing large numbers 

 of coloured corks into the river. Some of them were followed 

 for 8 miles, where the observation stopped, and were seen even 

 6 months after they had been thrown in, showing how long 

 solids may remain, even in a rapid river. 



Mud banks can obviously form independently of the solids of 

 sewage, which in many cases amount to only a small portion of 

 their volume, or to merely a film on their surface. Besides the 

 presence of coal dust or soot, they often acquire a dark colour 

 from the production of black sulphide of iron, as described on 

 p. 115. Quantities of shrimps, mussels, and marine or fluviatile 

 organisms are usually found on them, which shows that they 

 are not poisonous to such life. 



(3) Destruction of fish and sometimes of aquatic vegetation. — The 

 two are connected, as the water plants not only themselves 

 serve for food for certain fishes, but also harbour the lower 

 forms of animal and vegetable life which help to support fish. 

 In the Severn, where the deposit is at all abundant, there is no 

 growth of aquatic plants, and *'the absence of a clean bottom, 

 supporting weeds, will most probably act injuriously on fish."^ 



"The absorption of oxygen and the evolution of not only 

 CO.2, but of other gases such as HgS, must act injuriously on 

 fish life. Indeed, Konig and Haselhoif have shown that carp 

 and tench are injuriously affected by 8 milligrammes of HSg 

 per litre."^ 



That greater care is necessary in securing the cleanliness of 

 shores and estuaries has been for some time obvious, from the 



^ Reports, vol. ii., 1902, p. 97. 



- Ibid., p. 106. Further authorities on this subject are Nitsche and Tharaud, 

 Weigelt, J. Konig, Hoppe-Seyler, Duncan, Saare and Schab {Vcrunretnigung der 

 Gewassey, vols. i. and ii., by Dr. Konig), U.S. Fish Commission, 1880, Bulletin, 

 vol. V. See also evidence given before R. Sew. Commission, Interim Report, 

 vol. ii., 1902, especially pp. 485-496. 



