388 
The Composition of Sewage. [July, 
its possible ingredients, indeed, such as proto-salts of iron 
(ferrous salts), alkaline sulphides, &c., are not merely useless 
to vegetation, but positively injurious. 
Tire sewage of districts engaged in the metallurgical arts, 
the manufacture of hardware, &c., contains little extra 
organic impurity. On the other hand, it often holds in so- 
lution iron in considerable amount, derived, e.g., from 
“ pickling ” iron wire. Copper and tin are rarely present, 
the value of their solutions being a guarantee that they will 
not be knowingly and wilfully run into the sewer. But the 
proto-salts of iron — e.g., ferrous chloride, formerly known 
as muriate of iron, produced whenever metallic iron is 
scoured or cleaned from rust by means of hydrochloric acid 
(muriatic acid, spirit of salts) — is abundant in certain kinds 
of sewage, and is not merely hurtful to crops for the time 
being, but is a permanent steriliser of the soil. 
The sketch of the sewage of manufacturing towns just 
given is of course very general. Each such town, in faCt, 
turns out a special quality of sewage, the nature and effects 
of which can be understood only after careful experiment 
and observation. 
Both in residential and in manufacturing towns there is 
generally a very marked difference between the day and the 
night sewage. In a residential town the sewage, from mid- 
night to 5 or 6 in the morning, is very much reduced both 
in quantity and in strength. Often it consists of little more 
than the surface-water from the streets and roofs, &c., and 
of the ground-springs, which often find their way into the 
sewers to a considerable extent. As the day advances the 
flow of sewage becomes more abundant and more offensive, 
and is at its worst from 2 to 8 p.m., or thereabouts. In 
manufacturing towns the difference between the day and the 
night sewage is less marked, since certain kinds of waste 
waters are discharged in the night if requisite, and refuse 
which should not rightfully be run into the sewers at all is 
often introduced when there is the least chance of observa- 
tion. Hence it is a serious mistake to imagine that the 
night sewage may be safely neglected, and allowed to pass 
into the water-courses without purification. 
On Sundays in a residential town the sewage differs little, 
if at all, from its condition on the other six days of the 
week. In manufacturing towns it is on Sundays more 
purely excrementitious in its character, the industrial waste 
waters being in great part wanting. The same holds good 
with respect to public holidays. In small agricultural towns 
the sewage on market-day is very perceptibly stronger 
