458 NUTRITION ANIMAL HEAT AND FORCE. 



is produced. The weight may be supported at the height to which it has 

 been raised, for an indefinite time ; but it still possesses the potential energy 

 which has been imparted to it, and when the support is removed, this poten- 

 tial energy is converted into force which may be converted into heat. Poten- 

 tial energy may be converted directly into heat, as when a body is oxidized. 

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

 produced, for all force may be converted into heat. This conversion into 

 heat, directly or indirectly, affords a convenient measure of potential energy. 

 Using the example of the change of potential energy into heat by oxida- 

 tion, 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 esti- 

 mated by calculating the heat produced by the body falling. 



If the entire body of an animal were 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 can imagine an animal perfectly 

 quiescent, neither losing nor gaining weight, nourished by food, expending 

 no force in circulation and respiration, but supplied with oxygen, the poten- 

 tial energy of the food could be measured 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 be 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 be 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, circu- 

 lation 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 formulate'd 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 transmitted to another body of 

 equal weight, without any expenditure of energy in friction, it would raise 

 the second weight to an equal height. The arbitrary unit of this force is a 

 foot-pound or a kilogrammetre, terms which have already been defined. 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 would exactly equal the falling force. A heat-unit is therefore said 

 to be equal to a definite number of foot-pounds or kilogrammetres. Cal- 



