, straw or grain, there will be no
silica in her solid excrements, but there will be phosphate of lime
and magnesia. Her fluid excrements will contain carbonate of potash
and soda, together with compounds of the same bases with inorganic
acids. In one word, we have, in the fluid excrements, all the
soluble parts of the ashes of the consumed food; and in the solid
excrements, all those parts of the ashes which are insoluble in
water.

If the food, after burning, leaves behind ashes containing soluble
alkaline phosphates, as is the case with bread, seeds of all kinds,
and flesh, we obtain from the animal by which they are consumed a
urine holding in solution these phosphates. If, however, the ashes
of food contain no alkaline phosphates, but abound in insoluble
earthy phosphates, as hay, carrots, and potatoes, the urine will be
free from alkaline phosphates, but the earthy phosphates will be
found in the faeces. The urine of man, of carnivorous and
graminivorous animals, contains alkaline phosphates; that of
herbivorous animals is free from these salts.

The analysis of the excrements of man, of the piscivorous birds (as
the guano), of the horse, and of cattle, furnishes us with the
precise knowledge of the salts they contain, and demonstrates, that
in those excrements, we return to the fields the ashes of the plants
which have served as food,--the soluble and insoluble salts and
earths indispensable to the development of cultivated plants, and
which must be furnished to them by a fertile soil.

There can be no doubt that, in supplying these excrements to the
soil, we return to it those constituents which the crops have
removed from it, and we renew its capability of nourishing new
crops: in one word, we restore the disturbed equilibrium; and
consequently, knowing that the elements of the food derived from the
soil enter into the urine and solid excrements of the animals it
nourishes, we can with the greatest facility determine the exact
value of the different kinds of manure. Thus the excrements of pigs
which we have fed with peas and potatoes are principally suited for
manuring crops of potatoes and peas. In feeding a cow upon hay and
turnips, we obtain a manure containing the inorganic elements of
grasses and turnips, and which is therefore preferable for manuring
turnips. The excrement of pigeons contains the mineral elements of
grain; that of rabbits, the elements of herbs and kitchen
vegetables. The fluid and s