re, produced a considerably
larger crop than 240 lb. of phosphoric acid in the sludge. That is to
say, that the phosphoric acid in the sludge did not exert more than
one-fifth of its theoretical effect. The explanation of this somewhat
strange result Dr Munro finds in the unsuitable physical character of
the sludge-cakes. In farmyard manure we have a loose texture and a large
amount of soluble constituents when well rotted. It thus quickly
distributes its fertilising elements throughout the soil. In the case of
the sludge, on the other hand, its composing particles are closely
compacted together, and thus offer the greatest resistance to mechanical
and chemical disintegration. "As a matter of fact," says Dr Munro, "the
sludge-plots in my experimental series were all readily identified, when
the roots were pulled, by the presence of unbroken and undecomposed
clods of cake, which had evidently given up, at most, a small portion of
their valuable ingredients to the soil."

Briefly stated, therefore, the objections to chemical precipitation as a
means of dealing with sewage are these--viz., that while it relieves
sewage of all its organic matter, and to a large extent of its
phosphoric acid, it fails to extract any ammonia, which is thus lost;
that the resulting sludge is consequently so poor in fertilising matters
as scarcely to make it worth while to remove it any distance for
manuring purposes; and that, further, owing to its unfavourable
physical character, as at present made, even the small percentage of
plant-food it contains is not realisable, within, at any rate, anything
like a reasonable time, to its full theoretical extent.

The most profitable method of treating sewage must be determined by
various local conditions; and it must be clearly understood that the
question of sewage disposal is primarily a sanitary one, and that it
must be dealt with from the sanitary aspect. The most profitable way of
applying sewage as a manure, however, will doubtless be found by
combining chemical precipitation and land irrigation.

CHAPTER XVIII.

LIQUID MANURE.

The adoption of irrigation as a means of utilising sewage, suggests a
short consideration of the value of liquid manures. It has been a custom
on many farms to apply the liquid manure got from the oozings of
manure-heaps, the drainings of the farmyard, byres, stables, piggeries,
&c., directly to the soil. Indeed, so strongly has the belief in the
superiorit