Use of Town Seioage as Manure. 
1G3 
tlorizers, and for this purpose tlioy are very excellent. It is to 
be remembered that decomposing animal and vegetable matters, 
in addition to the peculiar odours which distinguisli the putre- 
faction of each kind of substanc^e, evolve two general products — 
sulphuretted hydrogen and ammonia — generally in combination 
with each other. Sulphuretted hydrogen is itself a highly 
offensive and injurious gas, but ammonia when pure is by no 
means disagreeable ; but it would appear that ammoniacal 
vapours have a singular tendency to act as vehicles or carriers 
of putrid odours, and the escape of ammonia is intimately con- 
nected with the offensive character of such effluvia. We have 
seen that many substances have the property, by virtue of the 
acid they contain, of arresting the escape of ammonia, and there- 
fore of materially diminishing putrid smells— such are gypsum 
and sulphuric acid. But ttie salts of zinc and iron not only com- 
bine with ammonia by their acid, but at the same time their other 
ingredient, the iron or zinc, effectually removes all smell of 
sulphuretted hydrogen by forming with it insoluble sulphurets. 
These salts are therefore very effective deodorizers, but they do 
not any more than the other agents of which we have spoken 
retain the ammoniacal compounds in solution in sewage. 
The use of compounds of magnesia in the preparation of 
sewage manure stands upon a different, and, theoretically, a 
much more favourable basis, than those which have been pre- 
viously mentioned ; and it is this : magnesia enters into the 
composition of one of the very few insoluble, or comparatively 
insoluble, compounds of ammonia with which chemistry has 
made us acquainted. The sulphate, muriate, nitrate, carbonate, 
and so on, of ammonia, are all soluble salts ; it is, therefore, 
of no use to produce tliem by any addition of the sewage ; 
they flow away in the water and cannot be retained by mecha- 
nical filtration. But ammonia forms with phosphoric acid and 
magnesia a salt of comparatively small solubility ; and as the 
two latter ingredients exist in all forms of liquid manure, the 
addition of magnesia would seem an excellent method of securing 
both of these valuable elements at once. Some years since it 
was proposed (I believe by Dr. Angus Smith) to add a cheap 
salt of magnesia to putrid urine, which contains, as we have 
seen, ammonia in large quantity and soluble phosphates a 
precijiitate of phosphate of magnesia is produced, and there is 
little doubt that by this means a most valuable manure may be 
obtained. 
Very lately a patent has been taken for the use of burnt mag- 
nesian limestone in the manufacture of sewag-e manure. This 
variety of limestone occurs abundantly in the north and west of 
England, and would therefore not be an expensive material. It 
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