240 PRINCIPLES OF ANIMAL NUTRITION. 
The first method, when available, is obviously to be preferred, 
and is to be regarded'as indispensable in all exact investigations 
into the energy relations of the food of man or of animals. With 
materials whose proximate composition is fairly well known, how- 
ever, the agreement between the computed and the actual heat of 
combustion is very close, as has been shown by Wiley & Bigelow * 
and Slosson ¢ for hulled cereals and cereal products. Atwater & 
Bryant, in their publication just referred to, have discussed this 
question very fully in relation to human foods and have proposed 
a series of factors for the ingredients of the various classes of foods 
by whose use they obtain a most satisfactory agreement with the 
calorimetric results. 
On the other hand, in case of vegetable products containing 
much woody and fibrous material the actual heat of combustion 
is higher than that computed under the ordinary interpretation of 
the results of chemical analysis. Thus the actual heat of combus- 
tion of unhulled oats was found by Wiley & Bigelow to be about 
4.5 per cent. higher than the computed value, and Merrill { has 
obtained similar results for wheat middlings and bran and for hay 
and silage. This obviously arises from the presence among the 
ill-known bodies constituting the so-called lignin and incrusting 
substances of compounds having higher heats of combustion than 
the common carbohydrates. It is not impossible that a series of 
factors similar to those of Atwater & Bryant might be worked out 
for different classes of stock foods, so that their heats of combustion 
might be computed from their chemical composition. In view, 
however, of the comparative case and rapidity with which direct 
calorimetric results can be accumulated it may be doubted whether 
such an undertaking would repay the labor involved. 
Methods have also been proposed and somewhat extensively 
used for computing the heat of combustion of the digested portion 
of the food. This phase of the subject, however, can be more 
profitably considered later. 
The Energy of the Excreta.—For the visible excreta (feces 
and urine) substantially the same method is available as for the food, 
* Jour. Am. Chem. Soc., 20, 304. 
+ Wyoming Expt. Station, Bull. 33. 
} Maine Expt. Station, Bull. 67, p. 169, 
