DECREASE OF ANIMAL LIFE 383 



washings from writing-slate mills where, on the Seint and 

 Llyfni, the dust is said to form in the beds of the rivers a 

 layer as hard as cement. 



Other pollutions act as direct poisons to the animal life 

 of the stream. The waste water of bleaching works may 

 contain chlorine ; in that of paper mills have been found 

 chlorine and sulphur compounds, as well as free sulphuric 

 acid, to such an extent that a mixture of one part of waste 

 to nine parts of river water killed Trout and Perch in one 

 or two days. Distilleries poison streams with their organic 

 acids, woollen mills with the alkalis from their wool washings, 

 and gas-works with their carbolic acid, gaslime and cyanides. 

 To gas-works' poisoning, fish are particularly sensitive, 

 i part of waste in 200 parts of river water having been found 

 to render an American Brook Trout moribund in 10 minutes, 

 while other fishes survived only a few days. Waste water 

 from iron and steel works may contain acids and may 

 deposit a precipitate (ferric hydroxide) which adheres to the 

 gill-filaments of fishes and checks respiration. So deadly 

 are iron pollutions that experiment with waste water from 

 a Canadian nail works has shown that a dilution of i part 

 of the waste liquor in 1000 parts of the stream water may 

 kill fishes such as Smelt and American Brook Trout in the 

 course often minutes to half an hour. 



Water discharged from old coal mines may also prove 

 fatal to the inhabitants of a river, owing to the sulphuric 

 acid set free by the breaking down of iron pyrites in the 

 coal and accompanying strata. Such a pollution, accom- 

 panied by washings from a lime-kiln, recently found its 

 way into a small loch in Scotland, with the result that the 

 fishes were all killed off and the invertebrate life was seriously 

 reduced, but a year afterwards Fresh-water Shrimps which 

 had previously been scarce, reappeared and bred in great 

 numbers in the loch. 



Another frequent source of pollution inimical to animal 

 life, arises from the discharge into rivers of organic matters 

 such as sewage. Harmless as it may be in itself, sewage 

 almost invariably develops noxious properties, as deadly as 

 the poisonous disinfectants by which it is often accompanied. 

 During its decomposition, through the action of bacteria, 

 toxic substances may be developed, but even in their absence, 



