158 
Use of Town Sewage as Manure. 
in London at a price varying, I believe, from 21. to 3/. per ton. 
Wood charcoal is a more expensive and much less effective 
variety ; its deodorizing properties are smaller than those of 
peat charcoal, and I am not aware that it has been employed in 
any practical operations as a deodorizer. 
Other forms of charcoal have lately come into use for the 
purpose of preparing sewage manure. Tlicy are made by 
charring night-soil, or in some cases sewage matter itself pre- 
viously collected. The charcoal obtained by the distillation of 
bituminous shale has also been employed for the same purpose. 
In considering tlie effect of cliarcoal as a deodorizer, we must 
carefully discriminate between the different circumstances in 
which it is to be employed. 
Great misconception exists in regard to the powers of charcoal. 
Charcoal is known to liave the power of absorbing ammonia 
and other gases, and that to a very considerable extent ; it is 
taken for granted, therefore, that this power exists equally under 
all circumstances, but such is not the case. It absorbs ammonia 
by virtue principally of its great porosity — by a kind of surf ace 
attraction — an attraction possessed by all solids for gases, and 
having relation of course to the quantity of surface — hence 
charcoal, in common with all porous bodies offering in a given 
weight a large amount of surface, has a great absorptive power 
for gases. But this property is also possessed in an eminent 
degree by water — more especially is this the case where gases, 
largely soluble in water, are in question. When, therefore, we 
treat charcoal, saturated with ammoniacal gas, with water, the 
superior attraction of the water comes into play, and the am- 
monia is transferred from the charcoal to the solution. In the 
opposite case, that is to say, when a solution of ammonia is 
filtered through charcoal, its strength cannot be sensibly dimi- 
nished. Indeed this fact has been experimentally demonstrated 
by Dr. Anderson, who published a series of trials, proving the 
want of power in charcoal to remove ammonia from solution, 
and I have myself made many experiments of the same kind 
with a like result. It is, therefore, an entire mistake to suppose 
that the property of charcoal to absorb ammonia from an atmo- 
sphere containing it, extends also to the separation of this alkali 
and its salts from liquids. 
Charcoal has the property of separating some organic sub- 
stances from solution, and it might be thought that it would at 
least have this effect on the organic matter contained in sevvage. 
This particular power of charcoal is very limited, and only 
applies to a small class of substances. For instance in the opera- 
tions of the sugar-refiner a solution of brown sugar is filtered 
through animal charcoal. The colouring matter is removed, but 
