UTILIZATION OF FOOD 415 



Energy Balance. The energy balance may be determined in 

 two ways: 



First, the energy in the food fed, and in the solid and the liquid 

 excrement may be determined by direct measurements, and that 

 lost as marsh gas calculated. The heat value of the fat or flesh 

 gained or lost may be calculated from the loss or gain of fat and 

 flesh found by the nitrogen and the carbon balance. From these 

 figures we estimate indirectly the energy used by the animal body. 



Second, the determination of energy in the food and in solid, 

 liquid, and gaseous excrements is made as stated above, but the 

 energy given off by the animal is measured directly. This form 

 of experiment requires the use of a respiration calorimeter. 1 The 

 results of the two methods should, of course, agree, but the sec- 

 ond method is adapted to studying problems which cannot be 

 solved by the first. For example, the energy given off during the 

 time digestion is going on may be compared with that given off 

 when no digestion is going on. In experiments with human be- 

 ings, the extra energy given off during reading, work, etc., may 

 be determined. 2 



Respiration Calorimeter. A respiration calorimeter consists of 

 a respiration chamber arranged so that the heat given off by the 

 animal can be measured. The chamber is insulated so that as 

 little heat as possible may be lost or gained by it. The incoming 

 air is cooled to a uniform temperature, and the outgoing air is 

 cooled to the same extent. The heat given off by the animal is 

 taken up by water circulating through tubes inside the chamber. 

 The temperature of the water is taken before it enters the cham- 

 ber, and when it leaves. The volume of water and the difference 

 in temperature of the incoming and outgoing water being known, 

 the heat eliminated is easily calculated to calories.. Allowance 

 is also made for the heat changes involved in the condensation 

 of water on the sides of the chamber, and for evaporation during 

 the experiment. 



1 Armsby, Bulletin 104, Pennsylvania Station. Exp. Sta. Record 15, 

 P- 1033- 



2 For description of human calorimeter see Bulletin 63, Off. Exp. Station. 



