164 
Use of Town Sewage as Manure. 
is proposed to break up the burnt limestone into a suitable form 
to serve either by itself or mixed with charcoal, &c., as a filter for 
the sewage, the ammonia and phosphoric acid of which it is 
expected to retain in the insoluble form. I am very unwilling 
to express a decided opinion on this process : originated by a 
chemist of ability, it possesses the advantage of correct principle, 
to which none of the other methods can lay claim. In all those 
plans which have previously engaged our attention there is an 
absence of any effective cause for the retention of the soluble 
ammoniacal compounds : but in this process the deficiency is, 
theoretically at all events, supplied. I say theoretically, because 
there are circumstances which, it is to be feared, will seriously 
interfere with its success. To mention only one or two : phos- 
phate of ammonia and magnesia is not absolutely an insoluble 
salt; and it has already been shown that for eveiy part of am- 
monia present in sewage we have to encounter the solvent 
action of 10,000 parts of water, containing all sorts of other 
salts ; whether under these circumstances the insolubility of the 
compound produced will l)e sufficient to preserve it from being 
washed away, is matter of doubt. But further, this compound 
will have to encounter the formidable action of that universal 
solvent, the carbonic acid, contained in the sewage. It is true 
that this may be neutralized by a free use of the magnesian lime- 
stone, but not without the attendant disadvantage of introducing 
into the manure large quantities of useless carbonate of lime, as 
was before shown in the case of ordinary lime. 
But whatever may be the difficulties attending it, this process 
deserves consideration, and it is much to be wished that it may 
be worked out successfully. 
The last method to which the reader's attention will be directed 
is one suggested by myself, and open to many of the same diffi- 
culties as the otliers. It consists in the use of certain compounds 
of silica. My researches have established, as the readers of this 
Journal may remember, the existence of certain double silicates 
of alumina and of another base which may be either ammonia, 
potash, soda, magnesia, or lime. These double silicates are all 
more or less insoluble in water. The lime compound is decom- 
posed by salts of all the other alkalies and alkaline earths — for 
instance, double silicate of alumina and lime, when digested with 
sulphate of ammonia or sulphate of potash, is converted into a 
corresponding and sparingly soluble compound of ammonia or 
potash, whilst sulphate of lime remains in solution. The lime 
compound would, therefore, remove from solution substances of 
great agricultural value. 
The expense of these compounds would hitherto have pre- 
cluded their use for such a purpose ; but it happens fortunately 
