RELATIONS OF HEAT TO FORCE 409 



It is converted indirectly into heat, when movement, falling or other 

 force is produced ; for all force may be converted into heat. The con- 

 version into heat, directly or indirectly, affords a convenient measure of 

 potential energy. Using the example of the change of potential energy 

 into heat by oxidation, the energy stored up in matter is measured by 

 estimating the heat produced by oxidation as so many heat-units. Using 

 the example of falling force imparted to a weight, the potential energy 

 imparted to the body is estimated by calculating the heat produced by 

 the body falling. 



If the entire body of an animal were to be burned in a calorimeter, 

 the heat produced would be an exact measure of the potential energy 

 of the tissues converted into heat by oxidation. If one could imagine 

 an animal perfectly quiescent, neither losing nor gaining weight, nour- 

 ished by food, expending no force in circulation and respiration, but 

 supplied with oxygen, the potential energy of the food could be meas- 

 ured by the heat produced. In animal organisms, heat is produced 

 mainly by oxidation, although other chemical processes contribute to 

 the production of heat to some extent. The body contains the potential 

 energy stored up in its tissues. The oxygen taken in by respiration 

 changes a certain part of this potential energy into heat. If food is 

 not supplied in adequate quantity, the body loses weight by this change 

 of tissue into certain matters, such as carbon dioxide, water and urea, 

 which are discharged. Food supplies the waste of tissue and is the 

 ultimate source of the potential energy of the body. If food is supplied 

 in excess, that which is not in some form discharged from the body 

 remains and adds to the total potential energy stored up in the organism. 



Kinetic energy is mechanical force. It is the force of a falling body, 

 or as regards animal mechanics, it is muscular force used in respiration, 

 circulation or any kind of muscular work. In physics, kinetic energy, 

 or force, and heat are regarded as mutually convertible. The reasoning 

 by which this law was formulated is the following: 



The force used in raising a weight to a certain height, which is 

 imparted to the weight as potential energy, is precisely equal to the 

 force developed by this body as it falls. If this force could be trans- 

 mitted to another body of equal weight, without any loss of energy by 

 friction, it would raise the second weight to an equal height. The 

 arbitrary unit of this force is a foot-pound or a kilogrammeter, terms 

 that have already been denned. The falling of a body of a certain 

 weight through a definite distance produces a definite quantity of heat 

 that itself is capable of producing force ; and it is assumed that the heat 

 produced by a falling body, if absolutely and entirely converted into 

 force, would raise that body to the height from which it had fallen, or 



