1885* I The Composition of Sewage. 387 
with the microscope, it will be found coated with the above- 
mentioned filaments of textile matters, which effectually 
choke up its pores. If we attempt to filter sewage through 
coke, asbestos, gravel, sand, peat, or ordinary arable soil, 
the same difficulty is encountered, the rapid flow observed 
at first becoming sooner or later obstructed. Hence, as I 
shall endeavour to explain more fully below, filtration is 
practically useless even for freeing sewage from its sus- 
pended impurities. 
There is another constituent of sewage harmless in itself, 
but which seriously interferes with most processes of treat- 
ment. I mean the sand, gravel, and pulverised stone which 
are washed into the sewers by every heavy shower. I show 
elsewhere how this silt, &c., by keeping bad company, ac- 
quires offensive properties. Its inconvenience in every form 
of sewage treatment will be duly noticed. 
In a manufacturing town or district the sewage is of a 
much more complicated character, and is more unsightly, 
though not more dangerous to public health. Nay, some- 
times its purification is easier than that of the sewage of a 
purely residential town. 
The pollution reaches its greatest height where the textile, 
tinctorial, and chemical arts are carried on. Here we find 
in the sewage, in addition to excrementitious matters, &c., 
sulphuric, hydrochloric, and nitric acids, alkalies, soap-lyes, 
solutions of iron, zinc, tin, alum, copper, chrome, antimony, 
and arsenic, waste dye-liquors and spent dye-wares, glue, 
sizes, waste tan-solutions, &c. It must not be supposed 
that all these substances will be found in the sewage of a 
manufacturing town at one and the same moment. Many 
of them, indeed, neutralise and precipitate each other, — a 
circumstance on which is founded a simple process for 
dealing with liquid industrial refuse. But if we watch the 
flow of such sewage we shall find striking changes both in 
its colour and its odour, according to the kind of filth just 
emitted by one or other manufacturing establishment. I 
have often seen some agent proposed for sewage-purification 
fulfil all requirements for hours in succession, but on the 
sudden discharge of a new kind of impurity into the sewer 
it has been found not merely useless, but even injurious, 
actually intensifying the evil. 
The sewage of such towns, it may be observed, though 
containing a larger proportion of solid matter, both sus- 
pended and dissolved, than that of a residential town, is of 
far lower agricultural value — a point to be had in remem- 
brance in selecting a method for its purification. Some of 
