Use of Town Sewage as Manure. 
165 
that their discovery has been coincident with that of a new source 
of silica, which greatly facilitates the manufacture of all the salts 
of tliis acid. Tlie beds of soluble silica, at the base of the chalk 
hills, may ultimately indeed be of considerable importance in an 
agricultural point of view. 
.^\t would seem possible to employ one or other of the compounds 
mentioned above in such a manner that the ammonia and potash 
of the sewage might be retained in the solid state. 1 hus, for 
instance, double silicate of alumina and lime, when so used, 
would become a corresponding salt of potash or ammonia. The 
same difficulties would, however, attend the use of these salts as 
that of the magnesian lime, before discussed, and it is very 
doubtful whether they could be used to practical advantage. 
We may recapitulate very shortly the conclusions to which we 
have been led by an examination of this subject. We learn that 
of the fertilizing matters of sewage, by far the largest portion 
exists in the liquid state ; that the solid portion has not even the 
agricultural value of ordinary excrement, far less that of night 
soil, to which we are unthinkingly in the habit of comparing it ; 
that the liquid is so largely diluted with water, that any attempt 
to concentrate it is totally out of the question ; that the greater 
number of plans that have been proposed for the production of a 
solid manure from sewage are only so far valuable that they assist 
in the separation and filtration of the matter in suspension, which, 
as well as the liquid, they deodorize and render manageable. 
Thus we have seen that charcoal does not retain the ammoniacal 
or alkaline salts, and in no way adds to the value of the solid 
refuse except in rendering it inofiFensive and in assisting its desic- 
cation ; that lime is merely useful in the filtration and deodorizing 
of the sewage, whilst on the other hand it introduces into the 
solid manure a large proportion of useless matter ; that the salts 
of alumina, iron, and zinc, deodorize and coagulate, but nothing 
more ; and finally, that the processes which really are adapted to 
the separation of the soluble fertilizing matters of the sewage, 
namely, tliose in which salts of magnesia and compounds of silica 
are employed, may altogether fail of success on' account of the 
diluted character of the liquid and its other solvent properties. 
But there is a further difficulty, and one which may prove 
insurmountable. The manufacture of solid manure from sewage, 
to be successful, must furnish an article of such value as to bear 
the expense of carriage to a considerable distance. It is not in 
the immediate neighbourhood of towns, where stable manure and 
other fertilizing matters are abundantly available, that the pro- 
duct of the servers is most wanted, or would be most appre- 
ciated. The market-gardens in the neighbourhood of London 
and other large towns will always have the command of abund- 
