2po 
How, When, and Where is Sewage Injurious ? [May, 
seem a convenience to the manufacturer to discharge his 
waste waters into the river, but by so doing he renders it of 
little use, save as a sewer, to all establishments situate 
lower down the stream. The sewage even of a residential 
town, except in very small proportions, unfits a river for use 
in bleach-, dye-, print-, or colour-works. 
The question has been raised by a pseudonymous writer 
in “ Ashore or Afloat ” whether sewage is really detrimental 
to fish ? We must here, in the first place, remember that 
there are fish and fish. Not all species are alike in the con- 
ditions under which they can subsist, or flourish. But one 
point, at least, is analytically demonstrated. The greater 
the proportion of organic pollution in a stream, the smaller 
is the percentage of free oxygen in solution, such oxygen 
serving to oxidise — in other words, to burn up — the im- 
purities, and proving insufficient. Again, we know that 
certain fishes, e.g., the trout, can live only in well-aerated, 
highly oxygenated waters. Putting these two considerations 
together we can have little hesitation in pronouncing sewage- 
pollution as at least one of the causes which have tended 
to reduce the fish in our rivers. That in many cases, such 
as those of the Thames and the Clyde, other causes 
are at work, and especially steam navigation, is highly pro- 
bable. 
We have now to consider what are the compounds or 
principles which make sewage different from ordinary water 
and render it unfit for domestic purposes — drinking, cooking, 
washing, bathing, See., and for the use of cattle ? 
Foremost come the compounds of nitrogen. These are of 
four kinds. There is nitrogen in organic compounds — spoken 
of by analysts as “ organic nitrogen,” or “ albumenoid am- 
monia.” Such substances are albumen, gelatine, and, in 
general, all the complex liquid or semi-liquid bodies of ani- 
mal origin. These bodies are introduced into the sewage 
in the shape of blood, urine, pus, mucus, half-digested ani- 
mal food, as well as by certain vegetable products. All these 
substances pass very readily into intense putrefaction, and 
are not only exceedingly offensive, but serve as nutriment 
for those low forms of animal and vegetable life (microbia) 
which have been already mentioned as especially dangerous. 
The second state is urea, which forms a very large part of 
the solids held in solution in urine. Urea is not dangerous 
in itself, and in contact with a ferment which is never absent 
in sewage it is resolved into carbonate of ammonia. Urea, 
in sewage, may, therefore, be regarded as a mere transition 
compound. 
