470 PRINCIPLES OF ANIMAL NUTRITION. 
An approximate computation of the probable differences in the 
heat evolved by the fermentation, based on such data as are avail- 
able, gives as a result 0.159 Cal. per gram of starch, or somewhat 
more than one-half the difference in average utilized energy, viz., 
0.265 Cal. per gram. The data on which the computation is based, 
however, are too uncertain to allow us to attach very much value 
to the results, except perhaps as an indication that the supposed 
cause of the difference in the utilization of the energy is insuffi- 
cient to fully account, for the effect. 
ConcLusions.—It cannot be claimed that the above results are 
sufficiently extensive or exact to permit final conclusions to be 
drawn, but their general tendency seems to be in favor of the hy- 
pothesis that the proportion of energy utilized is substantially inde- 
pendent of the quantity of food, provided that the changes in the 
latter are not so great as to modify the course of the fermentations 
in the digestive tract. The results upon starch just considered 
seem to indicate that if the variations in quantity or make-up of 
the ration are pushed beyond that point, a difference in the pro- 
portion of the energy utilized may be caused by a difference in the 
digestive work; in other words, that it is the availability that is 
modified rather than the proportion of the available energy which is 
recovered as gain. While not denying that the latter function may 
be also modified, either directly as the effect. of varying amounts 
of food, or indirectly by changes in the chemical nature of the sub- 
stances resorbed from the digestive tract under varying conditions 
of fermentation, it seems probable that the main effect is that upon 
availability. 
It is to be observed that the rations used in these experiments, 
while not heavy fattening rations, still produced very fair gains. 
The experimental periods were comparatively short and hence 
the testimony of the live weight itself is liable to be misleading. 
Taking the actual gains of fat and proteids as shown by the respi- 
ration experiments, however, and comparing them with the compo- 
sition of the increase of live weight in fattening as determined by 
Lawes & Gilbert, it appears that the total gain per day was equiva- 
lent to from 0.9 to 2.5 pounds gain in live weight per day in the ex- 
periments on coarse fodder, while in those upon concentrated feeds 
the corresponding range is from 1 to 3 pounds. 
