The data upon which it is based are those derived from the observation of the amount of heat generated by the combustion of a definite quantity of food out of the body; which, it is affirmed with certain deductions, represents the quantity of heat evolved by the oxidation of the same food within the body; and as heat is the equivalent of muscular force or energy, that aliment which, in burning, gives off the most heat, must, it is supposed, necessarily be the richest in the production of animal motive power. Of course these conditions will, amongst others, be very considerably modified by the extent to which the processes of the animal economy, such as digestion, assimilation, &c., can liberate the elements of the food so as to become available as sources of this energy.

Were these processes perfect, all the carbon of the carbonaceous, as well as that of the nitrogenous constituents of the diet, after deducting the carbon which passes off as urea (one part of dry nitrogenous matter yielding about a third of its weight of urea) would be utilised and converted into heat-producing power. But even under these circumstances a considerable portion of this thermotic power would be expended in sustaining the internal movements of the body, such as respiration and the heart’s action, which it has been computed are daily maintained by a force capable of raising 600,000 pounds a foot high.

No wonder if, with such varying factors introduced into the problem, physiologists and physicists should differ so widely in their calculations; and that, whilst one inquirer believes that food practically yields only about half the force which, according to theory, it actually contains; another estimates it at only one fifth.

The following table by Dr Frankland shows the amount of force which different foods yield when burned. The results agree very closely with those theoretically given by Playfair and others.

Energy developed by one gramme, or one ounce of the following substances, when oxidised in the body.

Name of
Substance		Per cent.
of Water		1 gramme will equal
kil.-metres of energy.		1 ounce will equal foot-tons of energy, or in other words, would raise the under-given number of tons, 1 foot high.[276]
Beef (lean)70·560455·0
Veal (lean)70·949645·3
Ham (lean, boiled)54·471164·9
Bread crumb44·091083·0
Flour...1627148·5
Ground rice...1591145·3
Oatmeal...1665152·0
Pea meal...1598146·0
Potatoes73·042238·5
Carrots86·022020·0
Cabbage88·517816·2
Butter...3077280·9
Egg (white of)86·324422·3
Egg (yolk)47·01400127·0
Cheshire cheese24·01846168·5
Arrowroot...1656151·3
Milk87·026624·3
Sugar (lump)...1418129·5
Ale (Bass’ bottled)88·432830·0
Porter (Guinness’ stout)88·445541·5

[276] The amount of work done is generally estimated in this country as so many lbs. or tons lifted 1 foot. In France it is expressed as so many kilogrammes lifted 1 metre,—and called ‘the kilogrammemetre,’ as above.

“A table of this kind,” says Dr Parkes, “is useful in showing what can be obtained from our food, but it must not be supposed that the value of food is in exact relation to the energy which it can furnish. In order that the force shall be obtained, the food must not only be digested and taken into the body properly prepared, but its energy must be developed in the place and in the manner proper for nutrition. The mere expression of potential energy cannot fix dietetic value, which may be dependent on conditions in the body unknown to us. For example, it is quite certain, from observation, that gelatin cannot take the place of albumen, though its potential energy is little inferior, and it is easily oxidised in the body. But, owing to some circumstances yet unknown, gelatin is chiefly destroyed in the blood and gland-cells, and its energy, therefore, has a different direction from that of albumen. So also of the

potential energy, it is quite possible that all is not usefully employed. The tables of energy give broad indications, and can be used in a general statement of the value of a diet; but at present they do not throw light upon the intricacies of nutrition.”

ENFLURAGE. See Pommade.