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II uw t When , and I Vheve is Sewage Injurious 
27.3 
A gas invariably present in natural waters, as invariably 
absent in sewage, and found only in an exceptionally small 
piopoition in polluted rivers, is free, gaseous oxygen. In 
sewage it is consumed by adting upon the organic matter. 
This negative feature of sewage and of polluted streams ac- 
counts for no small part of their injurious aCtion upon the 
higher aquatic plants and animals. 
As regards hardness {i.e., the presence of salts of lime and 
magnesm) sewage differs little from the water supply of the 
dis 11(51. It is usually a little softer on account of the 
presence of small quantities of soap and washing-soda, and 
of the introduction of a considerable bulk of rain- water. 
i he sewage of a residential town has generally an alkaline 
reaction; that of a manufacturing town maybe slightly acid 
from various kinds of industrial refuse. 
it will be at once seen on considering the extreme com- 
plexity of sewage, and the unstable character of most of its 
components, that as it flows it must be continually changing 
ll -n C j a i a6ter ; that a s P ecimen taken in the heart of a town 
will differ greatly in its chemical properties and in its physi- 
ological aCtion from another specimen which has travelled 
along the sewers for the distance, say of two miles. These 
changes are most rapid and decisive where, as in certain 
manufacturing towns, the sewage has a temperature of 6o° to 
70° rahr. It is needless to say that organic matter suspended 
or dissolved in water at this temperature must undergo very 
rapid fermentations and other decompositions. Wherever the 
seweis are open, as at the grids or ventilation-holes, clouds 
of steam lise up, carrying with them a very sickenin 0- 
odour. & 
Heie, I may remark, is a serious flaw in the water-carriage 
0 sewage, as at piesent carried on. If there are no venti- 
latois the sewage-gas is liable, under certain very possible 
circumstances, to be forced back into the houses, over- 
coming the valves of the sinks and water-closets. 
If there are ventilators of the ordinary kind, i.e. gratings 
or trap-doors, along the streets, all the passers by a?e 
forced to inhale the fumes. In some towns, e.g., in many 
pai Is of London, these trap-doors are constructed in the foot- 
ways, and are opened for a certain time daily to reduce the 
pressure within. At such times it is a common sight to see 
a small ciowd collect around the opening, craning out their 
necks as if determined to lose no chance of inhaling the vile 
fumes ! 
So far I have been speaking of the ingredients of sewage 
found to be hurtful to men or other animals living near 
