Lecture on Town Setoage. 
463 
consists of useless earthy and organic sweepings, I think we obtain 
a Avrong idea of the concentration of the sewage by striking an 
average in which we incorporarte all the analyses made throughout 
a certain period of the year. If we rejected the abnormal results, 
we shoxild arrive at a better idea of the average quantity of solid 
fertilising matter, which is our chief object. We should then find 
that the average proportion of solid matter, which is given by Dr. 
Hoffman and Mr. Witt as high as 102 grains in the imperial gallon, 
is too high, and that 70 grains per gallon, or one part in a thou- 
sand, is a fairer estimate than even the 86 grains, Dr. Letheby's 
average result. 
This quantity agrees remarkably well with observations that 
have been made in other localities. Thus, in the sewage of Bir- 
mingham I find an average of nearly 70 grains ; sometimes a little 
less, sometimes a little more. We must naturally expect such 
variations. Dr. Wrightson also found there about 70 grains ; and 
in the sewage of other towns the average of solid matter is seldom 
much higher than 80 or 90 grains. Even in the most concentrated 
sewage of Birmingham the amount of solid matter is, as I know, 
seldom higher than 105 grains. On the whole, then, I believe we 
are not far wrong in stating that town sewage contains on an average 
one part of solid matter in a thousand. 
Let us inquire, in the next place, into the character of the solid 
matter. Messrs. Hoffman and Witt estimated that the 102 grains 
which, according to them, are found in the imperial gallon, consist 
of — nitrogen G*7 ; phosphoric acid, 1"8 ; potash, 1 ; organic matter, 
o0'7 ; or a total quantity of 40'2 grains of fertilising matter — the 
remaining 62 grains being worthless. Supposing a gallon of London 
sewage to contain 90 grains of solid matter — an over-estimate, 
which 1 take that I may be certain of dealing fairly with sewage — 
the following Table will fairly represent its composition : — 
AvEKAGE Composition of Londok Sewage. 
1 Gallon 
contains 
1 Ton contains 
1 Ton of tbe 
Dry Constituents 
of Sewage 
contains 
Organic matter and salts of ammonia . . 
Yielding ammonia .. .. 7 grains 
Containing — 
Phosphoric acid 1 grain 
Wortliless matters .. 56 grains 
Total amount of constituents .. 
Grains. 
30 
60 
lbs. ozs. lbs. 
1 
0 3i 
2 
0 OJ 
0 1* 
1 li 
3 
lbs. lbs. 
747 
163i: 
1493 
23 
69 
140 
2240 
• 90 
I find then, in these 90 grains, 30 of organic matter (including 7 
grains of ammonia) and 60 of mineral matter, and that in this 
mineral matter the valuable portion, the phosphoric acid, amounts 
