464 Abstract Report of Agricultural Discussions. 
to 1 grain, and the potash to, at the maximum, 3 grains. In a ton of 
sewage we have 8? lbs. of salts of ammonia, 2 of mineral matter, 
and in this mineral matter half an t)unce of phosphoric acid, 1 J of 
potash, and nearly 2 lbs. of worthless matter. A ton of the dry 
constituents of sewage contains 163i lbs. of ammonia, 23 lbs. of 
phosphoric acid, 69 lbs. of potash, and nearly two-thirds of it is 
worthless matter. So that, even if -wo evaporate sewage into a 
state of dryness, we should still have in the solid matter a very 
considerable portion of useless material. This point deserves special 
attention, for in valuations of the sewage of towns it is always 
compared with Peruvian guano. Now, if we leave the water out 
of consideration, it is hardly fair to compare the dry matter of the 
sewage with a material like guano, which hardly contains any 
valueless substance. 
Let us now examine the value put upon sewage by various 
chemists. Professor Holfman calculates that a ton of sewage is 
worth on an average about 2c?., or 17s. Id. per 100 tons. Accord- 
ingly the whole sewage of London would be worth 3796/. per diem, 
or the enormous sum of 1,385,540/. per annum. Guano at 11/. per 
ton is the standard on which these calculations are based. It is 
calculated how much ammonia occurs in the solid matter of sewage, 
and this is valued at 56/. a ton. The amount of phosphate of lime 
is calculated at 11. a ton, and the potash at 3lZ. a ton; the result 
being that the total solid residue from sewage is thus valued, in 
round numbers, at 6/. per ton. Now, following the same track 
which other chemists have trodden, I find that, by taking the 
average composition which I here assume, the solid matter in London 
sewage would be worth about 5/. 2s. 4c?. a ton. In this estimate I 
take ammonia at 6(?. a lb., potash at 3c7. a lb., and phosphoric acid 
at 2c?. a lb. According to this estimate a ton of sewage would be 
worth not quite If J. 
These theoretical calculations, however, are altogether fallacious ; 
for, in calculating the value of a manure, we must not merely esti- 
mate the amount of fertilising matter which it contains, but must con- 
sider its bulk and combination. The calculations on which compari- 
sons are drawn between guano and sewage start on wrong premises. 
In guano we have a portable manure which we can supply when and 
where we want it, so as to supj^ly an abundance of food to certain crops 
like our root crops at a critical stage of their existence. The same 
quantity of guano or superphosphate mixed up with a large body of 
soil — say 18 inches deep — would have been of little service for such an 
object. When once the roots are fairly established, with their various 
fibres drawing nourishment from the soil, and their leaves sjiread to 
the sun and air, and thus the apparatus for taking in food on all sides 
is formed, the natural sources of supply are amply sufiScient to 
provide for their luxuriant growth. We cannot, in fact, materiallj^ 
alter the composition of our soils, taking the whole bulk of the 
soil into consideration, by any amount of manure. Nor can we, 
chemically sjoeaking, deteriorate the land by the most exhaustive 
crops, if we regard the soil as a whole. In reality we manure only 
