156 STORRS AGRICULTURAL EXPERIMENT STATION. 
combustion of the food which was available to the body. ‘The 
dried residue of the corresponding urine yielded 1,118 cal- 
ories, so that the net energy which was made available to 
the body from the food was 19,214 calories. In other words 
the heat of combustion of the total food was 21,384 calories., 
That of the total available food was 20,332 and that of the net 
available food, which represents its actual fuel value, was 
19,214 calories. 
In this view the actual or net fuel value of the food is the heat 
of combustion of that portion which is actually oxidized (or 
capable of being oxidized) in the body. It is the heat of com- 
bustion of the total food less the sum of the heats of combus- 
tion of the unoxidized residues of feces and urine; the very: 
small amounts of oxidizable materials in the products of res- 
piration and perspiration and the volatile organic compounds 
eliminated by the intestine being left out of account. For the 
carbohydrates and fats the actual fuel values are the same as 
the heats of combustion of the amounts actually available to 
the body for building and repair and for the yielding of kinetic 
energy. These amounts are those commonly called ‘‘ digesti- 
ble’’ and estimated as equivalent to the total amounts in the 
food less those eliminated in the undigested food residues and 
the metabolic food products of the feces. ‘The actual or net 
fuel* value of the protein is the heat of combustion of the 
available (so-called ‘‘ digestible’? ) protein less the heat of 
combustion of the unoxidized residues of protein excreted in 
the urine. 
The methods used for the determination of the heat of com- 
bustion of the urine deserve special mention, and will be dis- 
cussed in another place. It will suffice to say here that the 
urine was evaporated on the so-called filter blocks of cellulose 
described by Kellner.* 
DIGESTIBILITY VS. AVAILABILITY OF FOOD: MATERIALS. 
Frequent reference has been made in previous Reports to 
~ the fact that this method of experimenting does not show the 
proportions of a food material actually digested, since the feces 
contain, in addition to the undigested residue of the food, con- 
siderable quantities of other materials, the so-called metabolic 
products. ‘These latter consist chiefly of residues of the bile 

* Landw. Versuch-Stationen, Vol. 47, 1896, pp. 296, 297. 


