method immediately
about to be described--namely, precipitation--the amount of sewage the
soil is capable of purifying will be correspondingly increased. A
difficulty which may also be pointed out in connection with irrigation
as a means of disposing of sewage, is the impossibility of carrying it
on during frosty weather, when the land is frost-bound. In warm climates
irrigation has much to recommend it as a means of sewage disposal. In
damp and cold climates, on the other hand, there are many objections.

_Treatment of Sewage by Precipitation, &c._

We now come to consider the methods grouped under this second heading.
Mechanical filtration, of course, only aims at purifying sewage to the
extent of removing all insoluble suspended matter which it contains.
Different substances have been used as filters, the most generally used
being charcoal. Charcoal mixed with burnt clay, gravel, sand, &c., has
also been used.

In chemical precipitation, however, we have a method which claims to do
more. Beyond the extracting of all solid matters in suspension, it
removes (at any rate most chemical precipitants do) nearly all the
phosphoric acid, which, next to the ammonia, is the most valuable
constituent the sewage contains. Of all precipitants, lime has been the
most universally used; and on the whole, it is perhaps the best, for it
is both cheap and obtainable almost anywhere. According to an analysis
by the late Professor Way, the difference in the percentages of
phosphoric acid, potash, and ammonia, before and after treatment with
lime, in a sample of sewage, was as follows:--

_Grains per Gallon._

Before. After.
Phosphoric acid 2.63 .45
Potash 3.66 3.80
Ammonia 7.48 7.50

From the above we see that while sludge caused by lime as a precipitant
contains nearly all the phosphoric acid, there is not a trace of the
potash or ammonia removed. Sulphate of alumina has also been used, both
alone and in conjunction with lime. The advantage claimed by it over
lime is, that the resulting precipitate is much less bulky. In other
respects, however, it does not seem to be any more efficient as a
precipitant. In the well-known A, B, C process, a mixture of alum, clay,
lime, charcoal, blood, and alkaline salts, in different proportions, has
been used. This mixture is said to extract, in addition to the
phosphoric acid, a certain proportion of the ammonia; but the amount