is
so small as scarcely to be worth considering.

Numerous other chemical substances have been used, alone and also in
conjunction with one another, such as perchloride of iron, copperas,
manganese, &c. All alike, however, have failed to do more than effect
partial purification,--the best results, it may be added, being obtained
when the sewage thus treated was fresh. With regard to the manurial
value of the resulting sludges, much difference of opinion has existed.
The small percentage of phosphoric acid and nitrogen they contain has
prevented them from being used to any extent as a manure, as their value
did not admit of carriage beyond the distance of a few miles. By the
introduction a few years ago of the filter-press, their value has been
considerably enhanced. The old method of dealing with the sludge at
precipitation-works was to allow it to dry gradually by exposure to the
atmosphere. The result, however, of leaving sewage-sludge with over 90
per cent of water in it to dry in the air, was to encourage the rapid
decomposition and putrefaction of its organic matter, so that in many
cases the decomposing sludge proved to be as great a nuisance as the
unpurified sewage itself would have been. By the use of Johnson's
filter-press, however, a sludge containing 90 per cent of water was at
once reduced to 50 per cent or even less. By this means the percentage
of its valuable constituents was very much increased, and the
sludge-cake, besides being much more portable, was neither so
objectionable nor so liable to decomposition as before.

_Value of Sewage-sludge._

As to the value of this sludge-cake as a manure, we are happily in
possession of some very interesting and valuable experiments by
Professor Munro of Downton Agricultural College. The sludge experimented
upon was that produced by sulphate of alumina, lime, and sulphate of
iron, and contained, after being subjected to Johnson's filter-press,
from .6 to .9 per cent of nitrogen, and over 1 per cent of phosphoric
acid. It was found that the benefit resulting from the application of
the sludge was far from what in theory might have been expected. The
experiments were made with turnips; and the results obtained with
superphosphate and farmyard manure respectively, in the same field and
under exactly the same conditions, were contrasted with those obtained
with sludge. Thus it was found that 53 lb. of phosphoric acid as
superphosphate, or 60 lb. as farmyard manu