Use of Town Sewage as Manure. 
159 
the sugar remains in solution ; and even the quantity of colouring 
matter that is thus removed is very small. In the case of sewage, 
even supposing it to be fresh, we could not expect charcoal to 
remove the urea, which is the soluble organic matter of most 
value, because charcoal is actually used to purify coloured 
solutions of urea, the latter remaining in solution. 
Charcoal is used in two ways in the preparation of sewage 
manure. In the first, it forms a filter-bed through which the 
sewage is passed. In the second, it is mixed either with the 
liquid in tanks — the whole being mechanically filtered — or the 
sewage is first filtered through wire gauze, perforated zinc, or 
coarse clotlis, and the charcoal is added to the pulpy mass, to 
deodorize it and facilitate its drying. The first of these has the 
advantage of employing it both as a deodorizer and a mechanical 
filter; but in whatever way it is used, it must be clearly under- 
stood that charcoal removes nothing worthy of notice from actual 
solution, and its functions are confined to the separation and 
preservation in a portable shape of the solid matter in suspension. 
The next substance which we have to consider in its applica- 
tion to sewage manure is lime. Sewage-water, especially as we 
find it in London sewers, filters with great difficulty, and it 
becomes very necessary to find some means of causing a ready 
separation of the liquid and solid parts. Lime to a considerable 
extent effects this, and its use has been made the subject of 
several patents. The lime acts in coagulating the sewage, 
probably by neutralizing carbonic acid, which abounds in 
fermented sewer-water, and which holds in solution carbonate and 
phosphate of lime and other salts. A precipitate is thus formed, 
which encloses in it the light, floating organic matter, which 
would not otherwise subside. The operation, in fact, is strictly 
analogous to the clearing of beer or coffee by isinglass. The 
isinglass (gelatine) combines with the tannin of these liquids, 
and the flocculent precipitate encloses as in a net the floating 
particles in the liquid. 
It is obvious that lime when added to sewer-water will 
separate from solution all those substances which are held 
dissolved by acids. It has therefore the power to precipitate 
phosphate of lime, and to separate phosphoric acid from its other 
soluble combinations ; it also separates from solution certain 
organic matters with which it forms compounds, and the advo- 
cates of the use of lime in treating sewage have laid great stress 
on this property ; but in truth the quantity of material of any 
value to agriculture so precipitated is very small, and any advan- 
tage supposed to be gained by it is far more than counterbalanced 
by the large quantity of lime that comes to be present in the 
manure produced. I have examined, from time to time, many 
samples of the manure made from sewage by the addition of 
