Use of Town Sewage as Manure. 
161 
It is almost unnecessary to say that lime can in no way help 
in the preservation of the ammoniacal salts of the sewage. 
I pass on now to gypsum, — the office and properties of which 
are generally just as much misconceived as those of charcoal. 
Gypsum is, as is well known, a compound of sulphuric acid 
and lime. When this salt is brought in contact with the volatile 
carbonate of ammonia in the air, a cliange takes place, and sul- 
phate of ammonia (a non-\o\a.\.\\e salt) and carbonate of lime are 
produced. This is the way in which it acts when used to 
remove the ammoniacal vaj)oursof stables, and it is said to the 
ammonia. But the sulphate of ammonia is a soluble salt, like 
the carbonate or any other of its compounds, and water will 
wash it away with equal certainty. If then I filter sewage- 
water through a bed of gypsum, in the hope of stopping tlie 
ammonia, I do but deceive myself. I am at the most only con- 
verting one soluble salt of ammonia into another equally soluble, 
and not a portion of it is left behind. Gypsum, like charcoal, 
is a good substance to use as a deodorizer for the ultimate 
manure, but it cannot, in the smallest degree, increase its value 
by retaining the soluble ammoniacal compounds of the sewage. 
Gypsum, when added to undiluted urine, produces a preci- 
pitate containing a large quantity of phosphates, and, no doubt, 
of considerable value as manure. In this case the phosphates 
of soda, ammonia, &c., contained in urine are converted into 
insoluble phosphate of lime ; in the case of sewage, however, it 
is to be anticipated that the large quantity of carbonic acid present 
would effectually prevent the precipitation of phosphate of lime 
by the use of gypsum. 
Clays, burnt and unburnt, have been proposed as filters for 
sewage, and the researches which I have had the pleasure of 
publishing in this Journal on the absorptive properties of these 
substances have been quoted as justifying their use; but my 
experiments in no way warrant such a conclusion, and, in fact, 
especial pains were taken to prevent them from being so under- 
The following is the composition of manure made from London sewage by 
precipitated sulphate of alumina, lime, and charcoal : — 
Moisture 36*20 
Organic matter, charcoal, &c. . . . 19' 6.5 
Sand and other siliceous matter . . . 10*47 
Oxide of iron and alumina .... 4"31 
Phosphate of lime 2*63 
Hydrated sulphate of lime .... 5'89 
Carbonate of lime 20 '3.5 
Alkaline sulphates and muriates . . . 0-50 
100*00 
Nitrogen 0-62 
Equal to ammonia 0-75 
VOL. XV. JI 
