so much depends on various conditions, especially
the food.[133] The most reliable method of studying this question,
therefore, is to study it in its relation to the food consumed. Wolff
has calculated from numerous investigations that, with regard to the
amount of solid excreta produced by the food, the following percentage
of _organic matter_, _nitrogen_, and _mineral substances_, originally
present in the dry matter of the food, is voided in the dung:--

Cow. Ox. Sheep. Horse. Average.

Organic matter 39.5 42.5 44.0 44.1 42.5
Nitrogen 47.5 33.9 46.7 32.4 40.1
Mineral substances 53.9 64.6 57.9 62.5 59.7

There is one fact to be borne in mind in estimating the manurial value
of the dung of different animals--viz., that the quantity of dung voided
by one animal is much greater than that voided by another. Thus the
amount voided by the cow, for example, is much greater than that voided
by the horse; so that, in this way, the inferior quality of the former
is, to some extent, compensated for by its greater quantity.

2. _Urine._

The solid excreta possess, however, very much less manurial value than
the urine. The former, as already stated, are undigested
food-substances: any fertilising matters which they contain are such as
have failed to be digested or absorbed into the animal system. The
urine, on the other hand, contains those fertilising substances which
have been digested.

The amount of nitrogen and mineral matter, however, in the urine, does
not represent necessarily the total amount of these substances. Thus, in
the case of a growing or fattening animal, there is always a certain
amount of these substances being absorbed to build up the animal tissue
and put on flesh.

In this respect it will be seen that the composition of urine will vary
in the same way as that of the dung. In the case of the urine, however,
there is a compensating influence to be taken into account. Urine is a
waste product, and there is more waste in a young than in an adult
animal.

Another very important condition which determines the composition of
urine is the nature of the food, especially the quantity of water drunk.
This, of course, is obvious: the more water drunk, the poorer must the
composition of the urine be. But here again, as in the case of the dung,
this is largely compensated for by the total quantity voided--the more
dilute t