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until I assumed nothing more did come off (and in actual 
practice it was not at all difficult to be quite sure), then I 
titrated the barium-hydrate solution as before. From the 
experiments thus made I am very strongly of opinion that 
determinations of the amount of feee carbon dioxide 
dissolved in river waters, are valuable indicators of the 
state of that river as regards organic pollution. I consider 
the Irwell the best possible example of the saturation of a 
water with the gaseous products of the decomposition of its 
carbonaceous constituents, and I am quite certain that it is 
absolutely necessary to remove at once the large quantity of 
sewage pollution from the river, so that the other organic 
matters, which are less easily oxydised, may have a chance 
of being changed and destroyed by further oxydation. 
Owing to the rapid falling movement of the river, from 
its source above Bacup, from an altitude of 1300 feet, to 
Manchester, which may be on the bed of the river, about 
150 feet above the level of the sea, there is a first-rate 
chance for an ordinary river to purify itself. It will be at 
once apparent on consulting the table “ C ” that the Irwell 
at Bury is HALF as much polluted as it is at Throstle Nest, in 
Manchester. Again, on consulting Table “ D,” it will be seen 
that the Irwell at the Salford Boundary is far purer than 
the Irwell at Throstle Nest. Making a calculation from the 
analytical data given in that table, it appears that the water 
at Throstle Nest contains 76 per cent more albuminoid am- 
monia, and 36 per cent more oxydisable organic matter than 
the same water as it arrives at the Salford boundary. How 
can this tremendous increase in pollution be accounted for ? 
It is almost entirely due to pollution of the Irwell by its 
tributaries, the Irk and the Medlock, the sewage being mostly 
that poured into the rivers by the Manchester sewers, because 
the sewage of Salford has been diverted from the Irwell 
altogether, I believe. On referring to table “ D ” it will be 
