Some have expressed surprise that animals have refused to thrive, and have ultimately died, when fed upon animal jelly or gelatine (from bones) alone, nourishing though that substance as part of the food undoubtedly is. When given in sufficient quantity, gelatine might indeed supply carbon enough for respiration, with a great waste of nitrogen, but it is deficient in the saline ingredients which a naturally nourishing food contains.

Even on the natural mixture of starch and gluten in fine white bread, dogs have been unable to live beyond 50 days, though others fed on household bread, containing a portion of the bran—in which earthy matter more largely resides—continued to thrive long after. It is immaterial whether the general quantity of the whole food be reduced too low, or whether one of its necessary ingredients only be too much diminished or entirely withdrawn. In either case, the effect will be the same—the animal will pine away, and sooner or later die.

SECTION VII.—OF THE PRACTICAL AND THEORETICAL VALUE
OF DIFFERENT KINDS OF FOOD.

From what has been stated in the preceding section, it appears, that, for various reasons, different kinds of food are not equally nourishing. This fact is of great importance, not only in the preparation of human food, but also in the feeding of stock. It has, therefore, been made the subject of experiment by many practical agriculturists, with the following general results.

If common hay be taken as the standard of comparison, then to yield the same amount of nourishment with 10 lbs. of hay, a weight of the other kinds of food must be given, which is represented by the number opposite to each in the following table:—

Hay,10 Carrots,25to 30
Clover hay, 8to 10 Turnips,50
Green clover,45to 50Cabbage,20to 30
Wheat straw,40to 50Pease and Beans, 3to 5
Barley straw,20to 40Wheat, 5to 6
Oat straw,20to 40Barley, 5to 6
Pea straw,10to 15Oats, 4to 7
Potatoes,20 Indian corn, 5
Old potatoes,40? Oil-cake, 2to 4

It is found in practice, as the above table shews, that twenty stones of potatoes or three of oil-cake will nourish an animal as much as ten stones of hay, and five stones of oats as much as either. Something, however, will depend upon the quality of each kind of food, and upon the age and constitution of the animal. The skilful feeder of stock knows also the value of a change of food, or of a mixture of the different kinds of vegetable food he may have at his command.

The nutritive value of different kinds of food has also been represented theoretically, by supposing it to be very nearly in proportion to the quantity of nitrogen, or of gluten, which vegetables contain. Though this cannot be considered as a correct principle, yet as the ordinary kind of food on which stock is fed contains in general an ample supply of carbon for respiration, with a comparatively small proportion of nitrogen, these theoretical determinations are by no means without their value, and they approach in many cases very closely to the practical values above given, as deduced from actual trial. Thus, assuming that 10 lbs. of hay yield a certain amount of nourishment, then of the other vegetable substances it will be necessary, according to theory, to give the following quantities, in order to produce the same effect:

Hay,10  Turnips,60
Clover hay, 8Carrots,35
Vetch hay,[26] 4Cabbage,30 to 40
Wheat straw,52Pease and Beans, 2 to 3
Barley straw,52Wheat, 5
Oat straw,55Barley, 6
Pea straw, 6Oats, 5
Potatoes,28Indian corn, 6
Old potatoes,40Oil-cake, 2

If the feeder be careful to supply his stock with a mixture or occasional change of food, he may very safely regulate the quantity of any one he ought to substitute for a given weight of any of the others, by the numbers in the above tables—since the theoretical and practical results do not in general very greatly differ.