VENTILATION OF SEWERS AND DRAINS. 145 
evil. Having a base to start from: viz., the sewer, numerous 
projects were advanced for overcoming the difficulty. 
It is not my intention to discuss these in detail, except such 
as appear to have been effective in overcoming the evil, these 
however will be considered further on. 
Atmosphere of Sewers.—A diversity of opinion exists among 
scientists as to the chemical composition of the atmosphere of 
sewers. Under certain adverse conditions it has been termed 
“sewer gas,’ and under other conditions “sewer air.” ‘The former 
has proved injurious, if not fatal, to life, the latter is the air of the 
sewer, aud under favourable conditions may not greatly differ in 
its chemical composition from the air we breathe. Sewer gases 
are evolved from decomposing sewage—decomposition does not 
set up so rapidly where an abundant supply of water, a rapid flow, 
and free ventilation are available. 
The following table gives the result of analyses of ‘sewer air ” 
from sewers under different conditions :— 
cs} 
Atmosphere. Authority. aa © oa oe I 2 3 
on ce Weeedumer aie wt eco Ape 
fe) ba S n = <i 
Bad sewage,badly Per | Cent. 
decomposed air 
excluded ...| Letherby ... | 10°187| 15°89 08 | 73°83 
Choked sewer of 
Paris’ ..% ... G.deClaubry| 13:79 | 81:21 | 2°01| 2°99 
Ordinary sewer 
air, over sixty Bucks 
analyses ...| Hygiene | 19°95 | 79°69 | 0°86 traces |traces | traces 
In best of these... do. PAO TCIM ook 0:13 |traces |traces | traces 
London sewers ...| Letherby | 19°50 | 79°96 | 0°53 | ‘0016 
The analyses clearly shew the necessity of ventilation. In the 
case of the first test, it should be noted that it was the result of 
a laboratory experiment and after decomposition had purposely 
been allowed to take place—the sewage being allowed to stand 
for nine weeks. 
The second analysis shews the composition of gases evolved from 
an imperfect sewerage which had become what is termed a 
J—July 6, 1892. 
