2 72 How, When, and Where is Sewage Injurious ? [May, 
waste waters. These latter, however, rarely find their way 
into a stream unaccompanied by sewage in the stridt sense 
of the term. 
The absence of chlorine (common salt), say in veiy trifling 
quantity, may be accepted as a proof that a stream is fiee 
from animal pollution, but not necessarily that it is a safe 
drinking water. 
Soluble compounds of sulphur, especially hydrogen sul- 
phide (commonly known as sulphuretted hydrogen), ammo- 
nium sulphide, &c., are very generally present in sewage if at all 
stale, and are the main cause of its evil odour, though not 
always of its worst effedts. 
These sulphur-compounds are derived from the solid ex- 
crements of men and cattle who feed largely upon cabbages, 
turnips, and other vegetables containing sulphur, or from 
the water in which such vegetables have been boiled. 
Sulphuretted hydrogen is also formed in abundance when 
organic matter undergoes decomposition in presence, o, 
sulphates, such as gypsum. This, I remark in passing, 
is a reason against the use of gypsum (sulphate of lime 
or calcium sulphate) in any form, in the treatment of 
sewage. 
I know an instance of gypsum having been added to 
sewage-mud in order to aid in solidifying it. Sulphuietted 
hydrogen was given off in such torrents that the men em- 
ployed suffered from temporary blindness, — a well-known 
effedt of this offensive gas. 
Volatile compounds of phosphorus (hydrogen phosphide, 
phosphuretted hydrogen) have been popularly supposed to 
be given off by sewage and sewage deposits. I know of no 
analytical evidence in proof of this notion, and considei it 
as a mere freak of the imagination. 
Marsh-gas (light carburetted hydrogen or methane) is 
formed in abundance in stale sewage, as in sewers with too 
small a gradient, in ill-managed subsidence-tanks, and in 
accumulations of sewage-mud where decomposition is not 
arrested by proper chemical agents. If such deposits are 
stirred up and if a light is held over them the bubbles of gas 
ignite and burn with a very pale flame. Marsh-gas has no 
specially injurious adtion on the animal system, except 
when in such quantities as to exclude oxygen. 
Carbonic acid (carbon dinoxide) is another gaseous pro- 
duct of the decomposition of sewage and sewage mud, and 
is given off mingled with methane. Workmen have been 
suffocated by this mixture in ill-ventilated sewers and other 
under-ground passages. 
