AA1A1AL HEAT. 507 



communicates it to some other body during the process of bringing it 

 to rest. This is the fundamental form of energy. 



2. The other form of energy which a body may have depends not 

 upon its own state, but upon its position with respect to other bodies. 

 It is the energy possessed by a mass in consequence of its having 

 been raised from the ground. Potential energy can exist in a body, 

 all of whose parts are at rest. 



Rubner and Atwater have shown that the law of conservation of 

 energy is also applicable to the living body. The metabolism of the 

 food and tissues liberates their stored energy and converts it into 

 heat and motion. 



Radiant heat is one and the same thing as that which we call 

 light. When detected by the thermometer or by the sensation of heat, 

 it is called radiant heat. 



When equal weights of quicksilver and water are mixed together, 

 the resulting temperature is not the mean of the temperature of the 

 ingredients. The effect of the same quantity of heat in raising the 

 temperature of two bodies depends not only on the amount of matter 

 in the bodies, but also upon the kind of matter of which each is 

 formed. This is called capacity of heat, or specific heat. 



The capacity of a body for heat is the number of units required 

 to raise that body one degree of temperature. The specific heat of 

 a body is the ratio of the quantity of heat required to raise that body 

 one degree to the quantity required to raise an equal weight of water 

 one degree. 



Latent heat is the quantity of heat that must be communicated 

 to the body in a given state to convert it into another state without 

 changing the temperature. Avery describes it as follows: 



The latent heat of a substance is the quantity of heat that is 

 lost to thermometric measurement during liquefaction or vaporation, 

 or the amount of heat that must be communicated to a body to 

 change its condition without changing its temperature. 



The higher the temperature of a body, the greater is its radi- 

 ation. When the temperature of bodies is unequal, the hotter bodies 

 will emit more heat by radiation than they receive from the colder. 

 Therefore, on the whole, heat will be lost by hotter and gained by 

 colder bodies until thermal equilibrium is attained. 



The cause of heat is popularly explained to-day by what is known 

 as the "undulatory theory." According to this doctrine the heat of 

 a body is caused by an extremely rapid oscillating or vibratory motion 

 )f its molecules. The hottest bodies are those in which the vibra- 



