STANDARDS FOR RATIONS AND DIETARIES, 213 
and of the quantities of nutrients needed to supply it. But, un- 
fortunately, our knowledge in all these respects is still deficient, 
and furthermore the differences of individual animals of the same 
breed, to say nothing of the differences between animals of 
different breeds and species, are so wide that with the most per- 
fect knowledge of the laws of nutrition it will hardly be possible 
to set up accurate physiological standards for large classes of 
animals. “There is, however, the best ground to expect that a 
reasonable amount of experimenting of the right kind will bring 
us Satisfactory information in this respect. 
EXPERIMENTS NEEDED FOR PHYSIOLOGICAL STANDARDS. 
The experimental inquiry needs to be of two kinds; accurate 
experiments in metabolism, and practical feeding trials. For the 
first it will be essential to take account of the income and outgo 
of the body of the animal, as expressed, not simply in terms of 
food and drink on one hand and of excreta, solid, liquid and 
gaseous on the other; but also in terms of chemical compounds 
and especially of chemical elements of the income and outgo. 
These will give the data for the metabolism of matter in the 
body. For the most desirable results, however, it will be neces- 
sary to go a step farther and take an account of the income and 
outgo of energy. The income of energy will be the potential 
energy of the food; the outgo of energy will be the potential 
energy of the unconsumed food, z. ¢., that of the solid and liquid 
excreta, and the kinetic energy manifested in the heat diffused 
from the body and in the mechanical power of the muscular 
work performed.* In other words, we have to study the meta- 
bolism of energy as well as of matter. 
Experimenting of this kind is very costly. It requires elaborate 
apparatus, the most skillful assistants, and very long time. The 
respiration calorimeter which is being elaborated at Wesleyan 
University in connection with the work of this Station, is intended 
for inquiries of this especial kind. Of course it is not necessary 
that a respiration calorimeter or even a respiration apparatus 
shall be used in all experimenting in metabolism. Much can be 
learned from feeding trials in which the income and outgo of 
only a single chemical element as nitrogen is determined. But the 
respiration apparatus, and especially the respiration calorimeter, 
must be looked to for the most accurate and decisive results. 
* For detailed discussion of this particular subject see Bulletin No. 21 of the Office of 
Experiment Stations of the U. S. Department of Agriculture, entitled ‘*‘ Methods and Results 
of Investigations on the Chemistry and Economy of Food.”’ 
