SECT. XXVII. DYNAMICAL THEORY OF HEAT. 275 



heat produced by friction, whether the matter be solid or fluid, 

 is always in proportion to the force employed : in cold weather 

 we rub our hands to make them warm, and the harder we rub 

 the warmer they become. The warmth of the sea after a storm 

 is in proportion to the force of the wind ; and in Sir Humphry 

 Davy's experiment of melting ice by friction in the receiver of 

 an air-pump kept at the freezing point, the heat which melted 

 the ice was exactly proportional to the force of friction. This 

 experiment proves the immateriality of heat, since the capacity 

 of ice for heat is less than that of water. Thus mechanical 

 action and heat are equivalent to one another. Mr. Joule of 

 Manchester * has proved that the quantity of heat requisite to 

 raise the temperature of a pound of water one degree of Fahren- 

 heit's thermometer, is equivalent to the mechanical force deve- 

 loped by the fall of a body weighing 772-69 pounds through 

 the perpendicular height of one foot. This quantity is the 

 mechanical equivalent of heat. Thus heat is motion, and it is 

 measured by force. In fact, for every unit of force expended in 

 friction or percussion, a definite quantity of heat is generated ; 

 and conversely, when work is performed by the consumption of 

 heat, for each unit of force gained, a unit of heat disappears. For 

 since heat is a dynamical force of mechanical effect, there must 

 be an equivalent between mechanical work and heat as between 

 cause and effect. (N. 222.) 



Besides the temperature indicated by the thermometer, bodies 

 absorb heat, and their capacity for heat is so various that 

 very different quantities of heat are required to raise different 

 substances to the same sensible temperature. It is evident, 

 therefore, that much of the heat is absorbed and becomes 

 insensible to the thermometer. That portion of heat requisite 

 to raise a body to a given temperature is its specific heat, 

 but the latent or absorbed heat is an expansive force or 

 energy, which, acting upon the ether surrounding the ultimate 

 particles of bodies, changes them from solid to liquid, and from 

 liquid to vapour or gas. According to the law of absorption, 

 the transfer of heat from a warm body to one that is cold is a 



* This theory of heat and motion originated with Mr. Joule, of Man- 

 chester, who has maintained it with the greatest talent, both by experiment 

 and analysis ; and it has had an able advocate in Professor W. Thomson, of 

 Glasgow. 



