496 PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 



temperature of others in consequence of the absorption of heat rays, by 

 the hypothesis of an oscillating aether and atoms, on the other. 



If heat rays strike a body, a part of them penetrate into the sub- 

 stance and transfer the oscillatory motion of the heat waves of aether 

 either to the atoms of the aether within the body or to the atoms of the 

 body's substance. In the former case the body (for instance, rock salt) 

 permits the greater part of the heat rays to pass through it, and its 

 temperature is increased in a scarcely perceptible degree ; in the latter 

 case the body refuses almost entirely to let the heat rays pass, but its 

 temperature is noticeably heightened. It is then said that the body has 

 absorbed the heat rays. We conceive that in this absorption of the 

 heat rays the oscillating atoms of the aether waves or the rays strike the 

 atoms of the body, and in transmitting their own motion impart to them 

 an oscillating motion about their center of equilibrium, or they intensify 

 the existing vibration of the corporeal atoms. 



If, on the contrary, the oscillating material atoms of a body transmit 

 a part of their motion to the aether, we say that such a body radiates 

 heat. As with the aid of the law of radiatiou and absorption in regard 

 to radiating heat the propagation of heat by transmission may be re- 

 duced to a stratified radiation and absorption of heat, the increase of 

 temperature of bodies by transmission is based finally on an increased 

 motion of the particles of the warmer bodies. 



In order to be able to speak more precisely of that molecular motion which 

 is called heat, we must borrow from mathematical mechanics the following 

 important proposition : When bodies or their particles are in motion, 

 half the product of the mass so moved by the square of its velocity is 

 called the kinetic (dynamic or actual) energy or half of the vis viva. 

 This expression "living force" first used by Leibnitz (1(58(5), grew 

 into general use in mechanics and physics, while now it is more and 

 more replaced by the term " kinetic energy." The word " energy" was 

 applied in physics by Thomas Young (1807); but it was only recently 

 that Rankine fixed the definite conception of this word in the sense in- 

 dicated above. Energy is now understood to mean the capability of 

 doing work or of overcoming a resistance ; the kinetic energy is mathe- 

 matically equal to half the vis viva. In theoretical mechanics it is mathe- 

 matically demonstrated that work done by a body moved by a force is 

 equal to the increase of the kinetic energy experienced by the body 

 during the action of the force. 



We will first apply this proposition relative to kinetic energy, — to heat 

 as a molecular motion. It is conceived that the more kinetic energy 

 increases by means of the increased velocity of the oscillating particles, 

 the higher the temperature of the body will prove to be. In the devel- 

 opment of heat by friction, the kinet ic energy of the visible motion of the 

 bodies is turned into that invisible kinetic energy of the molecular mo- 

 tion which manifests itself as a powerful increase of temperature of the 

 bodies experiencing friction. In the increase of temperature of bodies 



