1016 EXPERIMENT STATION RECORD. 



are taken as equivalent to their potential energy, i. e., their value 

 for the production of heat and muscular work when they are burned in 

 the body, due allowance being made for the imperfect combustion of 

 the nitrogenous compounds. The same principle applies to the reserve 

 materials, principally fat and protein, which the body takes from the 

 food and makes part of its tissue before they are burned. 



In determining the balance of income and outgo of energy the income 

 is represented by the potential energy of the food. The factors of 

 outgo are the potential energy of the excretory products, the heat 

 radiated by the body, and the amount of muscular work performed. 

 These factors may all be expressed in terms of heat. The data for the 

 balance are usually expressed in units of heat, i. e., calories. The 

 potential energy of the food and excretory products is determined by 

 combustion in a bomb calorimeter or other suitable apparatus. Many 

 different dynamometers have been devised for determining the amount 

 of external work performed. Internal work (expended in chewing, 

 swallowing, and digesting the food, in the beating of the heart, etc.) 

 can not be measured directly, but it is believed that the energy involved 

 in this internal muscular work leaves the body in the form of heat. 

 Therefore this internal muscular work can be measured by determining 

 the heat given off by the body. This requires a special apparatus 

 called a calorimeter. When it is combined with a respiration apparatus 

 of some sort, as has been done in a few cases, it may be called a respi- 

 ration calorimeter. 



In stating the results of experiments in which the balance of energy 

 is studied it is of advantage to compare the energy of the food avail- 

 able for the body — that is, the net income — with the energy liberated by 

 the body. The gross income of the food is represented by its heat of 

 combustion. The net income is the gross income less the heat of com- 

 bustion of the undigested residue and metabolic products in the feces 

 and the unoxidized material in the urine. 



When the balance of income and outgo of energy is determined, if 

 the income exceeds the outgo, energy is stored in the body. It is 

 assumed that this is stored as potential energy in fat, protein, or other 

 reserve material. If the outgo of energy exceeds the income then 

 some of the material of the body has been broken down to liberate this 

 energy. 



Calorimetric experiments have a practical application in comparing 

 the energy available to the body from different foods. Further, calori- 

 metric experiments will afford a means forjudging of the accuracy of 

 respiration experiments and show more clearly what becomes of the 

 nutrients of the food, the carbohydrates as well as the fat, protein, and 

 extractives. Another application is in the study of the production of 

 work by draft animals. It is evident that the study of these problems 

 necessitates the measurement and comparison of the work performed 

 on different rations. Such experiments are also essential in studying 



