162 
Use of Town Sewage as Manure. 
stood. Clay, indeed, has the power to remove from solution 
almost every ingredient which is of any value in agriculture. 
But to what extent does this power reach ? Take, for instance, 
one ingredient of the sewer-water as before — the ammonia. In 
the experiments referred to, 1000 grains of a soil were found to 
separate from solution 3 grains of ammonia, or 3-lOtlis per cent. 
This was as favourable a result as any obtained. But of M'hat 
use would it be in the manufacture of manure ? In this case 
the misconception is one of quantity. The power of the soil 
to absorb manure is all-important in the circumstances under 
which it is naturally brought into play, that is to say, in the land 
itself, because there we have hundreds of tons of soil charged 
with the duty of retaining a few hundredweights of manure ; but 
it is quite another thing when we have to make a portable 
manure which shall be rich in all the elements of fertility. A 
soil fully charged with the soluble ingredients of sewage would 
of course, when brought to a proper state of dryness, be ex- 
tremely rich for all the purposes of vegetation, but it could not 
act as a manure unless it was put on at the rate of 20 or 30 tons 
an acre. I have indeed examined fertile soils containing as 
much as 3-lOths per cent, of ammonia, but no person would 
think of digging such earth to send it away for manure. 
It must be remembered that these remarks apply only to the 
effect of clay or soils, whether burnt or otherwise, on the soluble 
matters of sewage. If it is wished to use them as a mechanical 
filter, so be it ; but, except for absolutely local purposes, better 
substances are at our disposal. 
In the case of town sewage the employment of such means is 
clearly out of the question, for the carriage to and fro would 
cost far more than the value of the manure produced. 
The salts of alumina have been proposed for use in preparing 
sewage manure. They act solely in causing the coagulation of 
the sewer-water, and rendering its filtration possible. I would 
not, however, undervalue this effect, for in practice it is very 
important, if not indispensable ; the salts of alumina are perhaps 
the best that, with our present knowledge, can be employed for 
this purpose. When, to a quantity of London sewage, a small 
portion of lime is first added, and afterwards another small 
portion of sulphate of alumina, a flocculent precipitate is formed 
which quickly subsides, carrying down with it all suspended 
matter, and the liquid soon becomes bright and clear ; the sepa- 
ration by filtration is now an easy matter. I repeat that the 
salts of alumina, however valuable as an adjunct to filtration, 
are nothing more ; tliey do not practically separate ammonia or 
other fertilizing matters from solution. 
The salts of zinc and of iron have been largely used as deo- 
