THE FOOD AS A SOURCE OF ENERGY. 271 
posal of the organism without affording any clue to the use made 
of it by the latter, that is, to its availability in the more restricted 
sense. 
In actual investigation, of course, the metabolizable energy of 
the food is most accurately found by means of direct determinations 
of the heats of combustion of the food and the waste products. 
Except in the case of the intestinal gases no serious difficulties 
stand in the way of these determinations, and with the present im- 
proved and simplified methods of calorimetry it may fairly be 
expected that, in exact experiments, at least the energy of the food, 
feces, and urine will be directly determined, while it is not impossi- 
ble that more extended investigations than are now available may 
enable us to make, for different classes of materials, a fairly accurate 
estimate of the intestinal gases. As results accumulate from such 
investigations we shall gradually acquire a fund of information 
regarding the amount of metabolizable energy contained in foods 
and feeding-stuffs which it is perhaps not chimerical to suppose may 
one day largely take the place of our present tables of composition 
and digestibility. 
Up to the present time, however, but a comparatively small 
number of experiments upon domestic animals are on record in which 
the metabolizable energy of the food has been actually determined. 
In a somewhat larger number of cases the loss of energy in feces 
and urine has been determined, and in others that in the feces only. 
. As regards human food the data are somewhat more abundant, 
but nevertheless by far the greater part of our scientific knowledge 
of foods and feeding-stuffs is expressed in terms of (conventional) 
chemical composition and apparent digestibility. If, therefore, we 
would not forego the advantages which may be anticipated from a 
study, from the new point of view, of the accumulated results of 
the last half-century of experimental work in this domain, it is im- 
portant that we be able to estimate as accurately as may be the 
metabolizable energy of the food from its known or estimated com- 
position and digestibility. Not a little labor has been expended 
upon both aspects of the subject, particularly by Rubner in relation 
to the carnivora and man, by Atwater and his associates with rela- 
tion to human nutrition, by Kellner as regards ruminants, and by 
Zuntz and his associates in the case of the horse. 
