1885.J How, When, and Where is Sewage Injurious ? 26 g 
a district, the disease spreads on all sides and commits sad 
havoc, lhus persons who drink polluted waters hold their 
health or theii lives at the mercy of chance. 
But the microbia of contaminated waters find their wav 
also into the air, and may be inhaled with it. In every 
polluted uvei fermentation is constantly going on. If any- 
one carefully watches the Aire at Leeds, the Kelvin Water 
at Glasgow, the Irwell at Manchester, &c., he will see— 
especially if the day is warm and the barometer low- 
bubbles rise to the surface and burst. These bubbles con- 
sist ot sewage-gas, compounds generally of carbon and 
hydrogen, with more or less sulphur. But in bursting they 
carry up with them the disease-germs or microbia above- 
mentioned, which thus become diffused in the air. Whether 
these miciobia are also carried up into the air when polluted 
waters evaporate quietly, without the escape of bubbles, has 
been disputed, but may now be regarded as experimentally 
demonstrated. 
Iheie aie, of course, cases where the volume of the 
sewage poured into a river may be very trifling compared 
with that of the river itself. Even London could not suffice 
to pollute the Amazon. But most of the great cities of the 
modern world discharge their refuse into comparatively 
small streams. 
Seeing, then, that sewage is in the wrong place if poured 
into vvateis, we have to ask how does it behave on the land ? 
It is then spread out over a larger area than when it flows 
in a sewer or a river, and necessarily exposes a larger surface 
to evaporation. However porous the soil, and however 
complete the drainage, a quantity of the evil-smelling liquid 
rises up into the air, carrying with it disease-germs if such 
are present. It will be found that if water is allowed to 
flow, however gently, over the surface of dry soil, bubbles 
are formed and burst. Thus, whenever sewage is turned on 
to land after an interval, disease-germs will be carried into 
the air, just as in the case of the bubbles which form on a 
polluted river. Further, the putrescent matter and disease- 
germs will be absorbed by two-winged 4dies, and distributed 
over the food and the persons of human beings. 
Summing up this part of the subject, it may be safely 
asserted that sewage is harmful and offensive by its odour 
and its appearance, and especially by its affording a pabulum 
and breeding-ground for disease-germs. It is offensive in 
the water-courses and rivers by rendering the water unfit for 
human consumption, and for all delicate manufacturing pro- 
cesses. This last point is too often overlooked. It may 
