454 PHYSIOLOGICAL PHYSICS. [Chap, xxxiv. 



the amount of heat necessary to raise the temperature 

 of 1 pound of cold water 1 Fahr. is called the unit of 

 heat, so that the unit of heat is equal to mechanical 

 work measured by 772 foot-pounds. If, instead of 

 the Fahrenheit, the Centigrade scale be used, then the 

 amount of heat necessary to raise the temperature of 

 1 pound of water 1 0. is equivalent to 1,392 foot- 

 pounds mechanical work. The unit of heat, or 

 thermal unit, may also be measured on other 

 scales. Thus by French measure it would be the 

 amount of heat required to raise the temperature of 

 1 gramme, or 1 kilogramme, of water 1 C., and 

 could be estimated by its equivalent of mechanical 

 work necessary to raise so many grammes or kilo- 

 grammes so many metres high. 



In French measure the unit of heat, or calorie, 

 as it is called, is the quantity of heat necessary to 

 raise the temperature of 1 kilogramme of water from 

 to 1 C. Joule's equivalent becomes then in 

 French measure the work performed by raising 424 

 kilogrammes 1 metre high, which is expressed shortly 

 by saying that 1 calorie is equal to 424 kilogram- 

 metres. 



Heat is, then, a mode of motion, a form of energy. 

 When a falling body reaches the ground its energy is 

 not destroyed, it is only transformed ; the movement 

 of the body as a mass becomes transformed into 

 movements of the atoms of the body, one manifesta- 

 tion of which is the development of heat. 



Sources of heat. As has been seen, mecha- 

 nical work of all kinds may be sources of heat. 

 There are also chemical sources of heat ; wherever 

 chemical combination occurs, heat is produced. The 

 combination is due to affinity of two substances for 

 one another. So Tyndall represents the mutually 

 attracting atoms as "rushing together, and acquiring, 

 while crossing the insensible interval which separates 



