Purchased Food, and of its Residue as Manure. 209 
The soluble portion of the mineral food-constituents, consist- 
ing principally of common salt and potash-salts, is constantly 
rejected in the urine ; whilst the insoluble portion, consisting 
chiefly of phosphate of lime and magnesia, carbonate of lime 
and silica, passes away in the solid excrements. 
It is hardly necessary to state that no animal can live, for any 
length of time, exclusively upon starch or sugar, or upon 
albumen. The requirements of the animal body necessitate a 
mixed food containing all the constituents, to the functions of 
which brief reference has been made. With the exception of 
treacle, which is occasionally used for feeding purposes, there 
is no feeding-stuff which consists entirely or mainly of one 
group of alimentary matters. 
All feeding materials, whether they are cereal grains, legu- 
minous seeds, roots, grass, or chaff, are mixed foods, containing 
v ariable proportions of nitrogenous and non-nitrogenous organic 
matters, and of saline and phosphatic earthy compounds. In its 
natural state the animal eats no more than the necessary amount 
of food to provide, firstlv, carbon for the support of respiration 
and for keeping up the animal heat, and, secondly, enough nitro- 
genous and mineral constituents to keep in healthy action the 
complicated processes of digestion, assimilation, and secretion. 
A full-grown animal, in a state of perfect health, neither 
increases nor decreases in weight when it is allowed to help 
itself with as much grass, or whatever else may be its natural 
food, as it pleases, uncontrolled by human agency. The larger 
portion of the non-nitrogenous constituents of the food is oxidised, 
and passes off as carbonic acid from the lungs, w^hilst the mineral 
matters contained in the food are ejected from the system almost 
entirely, either in the urine or in the solid excrements. 
The nitrogenous constituents of food are decomposed more or 
less completely before they are ejected by the animal. As the 
result of this decomposition, two new classes of substances are 
produced. One class comprehends compounds containing all, 
or nearly all, the nitrogen of the decomposed albuminoids, 
united with comparatively little hydrogen, carbon, and oxygen ; 
the second class contains the remaining quantity of carbon, 
hydrogen, and oxygen. Urea, uric, and hippuric acids are the 
principal highly nitrogenised organic products ; lactic acid, fatty 
matters, and some other combinations of a less definite chemical 
character, are the products destitute of nitrogen which result 
from the decomposition of the albuminoids of food. 
Whilst thus the greater part of the non-nitrogenous con- 
stituents of food is wasted in the exhalations from the lungs, 
nearly the whole of the mineral and nitrogenous constituents of 
food pass into the solid and liquid excrements of animals. As a 
VOL. XII. — S. S. P 
