60 THE SALMON FISHERIES. 
such profitable utilisation by special treatment that their 
compulsory removal from our streams would more than 
pay the cost of the necessary works, without reference to 
the compensation that would be found in the improvement 
of the fisheries. 
An instance may be found in the plan adopted by the 
Devon Great Consolidated Mining Company for the 
extraction of copper in solution in mine-water, with most 
excellent results. The water is passed through large 
filtering beds, to divest it of all earthy matter, and then 
conveyed over scrap-iron, which throws the whole of the 
copper down. The iron is occasionally turned over and | 
brushed, to prevent it from becoming too thickly coated 
with copper, when it would cease to be acted on. The 
copper is caught in a catch-pit at the end of each water- 
course. The iron displaced by the action of the acid 
passes off in the form of ochre, which is caught in pits 
prepared for the purpose ; and the water is again finally 
taken through a series of catch-pits and ponds before being 
thrown into the river. 
The rivers of South Wales are all more or less seriously 
injured—many: of them utterly ruined—by the refuse from 
tin-plate works. This refuse consists of diluted sulphuric 
acid, holding iron in solution. In order to make iron plate 
or wire take the desired coating of tin, it is dipped in a 
“pickle” composed of a strong solution of sulphuric acid, 
which eats away all impurities and roughness from the 
surface. This highly poisonous liquid, when it has been 
used several times, becomes so fully charged with iron 
that it can do its work no longer; and so the manufac- 
turers, regarding it as an unprofitable servant, cast it 
disdainfully into the river, where it poisons the water, kills 
the fish—and anything else that happens to drink it— 
