SEWAGE OF LONDON AND OTHER LARGE TOWNS. 157 
Besides, we ought not to be satisfied with merely keep- 
ing the productive power of our agriculture from decline. 
With a rapidly increasing population the wisest course 
would be to reserve such supplies of guano as we may be 
able to obtain for the purpose for which nature appears 
to have designed it, that of forming a fertile soil where 
sterility at present exists. 
In concluding this part of my subject I would urge the 
importance of recollecting that in the estimate of fertility, 
regard should be paid not only to the weight of a crop but 
also to its nutritive value, determined in each case by che- 
mical analysis. Liebig states that an increase of animal 
manure gives rise not only to an increase of the number of 
seeds, but also to a most remarkable increase in the pro- 
portion of those nitrogenous substances which are the 
most important constituents of food. 
Having endeavoured to show the imperative necessity, 
I will say a few words on the means, of putting a stop to 
the present waste. The first step I conceive should be to 
prohibit the introduction into the sewage of any organic 
matter which can be avoided. For instance, scavengers 
should be constantly employed in collecting and removing 
horse dung from the streets. The present system of send- 
ing carts round at long intervals of time allows by far the 
larger portion of this manure to be washed by rain into 
the sewers, thus forming a very serious addition to their 
impurity. 
Then why should slaughter-houses be tolerated? If 
only meat slaughtered in the country were admitted into 
the town, I submit that the meat would be cheaper in 
regard to its intrinsic nutritive value. The distress suf- 
fered by the animals in their passage from the field to the 
town slaughter-house destroys the richness and flavour of 
their meat, even if it do not render it positively unwhole- 
