MOLECULAR CONSTITUTION OP BODIES. 437 



the velocity of each portion of the medium at any time can be expressed as 

 depending on the position and the time elapsed ; so that the motion of a medium 

 during the passage of a sound-wave is regular, and must be distinguished from 

 that which we call heat. 



If, however, the sound-wave, instead of travelling onwards in an orderly 

 manner and leaving the medium behind it at rest, meets with resistances which 

 fritter away its motion into irregular agitations, this irregular molecular motion 

 becomes no longer capable of being propagated swiftly in one direction as sound, 

 but lingers in the medium in the form of heat till it is communicated to 

 colder parts of the medium by the slow process of conduction. 



The motion which we call light, though still more minute and rapidly 

 alternating than that of sound, is, like that of sound, perfectly regular, and 

 therefore is not heat. What was formerly called Radiant Heat is a phenomenon 

 physically identical with light. 



When the radiation arrives at a certain portion of the medium, it enters 

 it and passes through it, emerging at the other side. As long as the medium 

 is engaged in transmitting the radiation it is in a certain state of motion, 

 but as soon as the radiation has passed through it, the medium returns to its 

 former state, the motion being entirely transferred to a new portion of the 

 medium. 



Now, the motion which we call heat can never of itself pass from one 

 body to another unless the first body is, during the whole process, hotter than 

 the second. The motion of radiation, therefore, which passes entirely out of one 

 portion of the medium and enters another, cannot be properly called heat. 



We may apply the molecular theory of gases to test those hypotheses about 

 the luminiferous aether which assume it to consist of atoms or molecules. 



Those who have ventured to describe the constitution of the luminiferous 

 aether have sometimes assumed it to consist of atoms or molecules. 



The application of the molecular theory to such hypotheses leads to rather 

 startling results. 



In the first place, a molecular aether would be neither more nor less than 

 a gas. We may, if we please, assume that its molecules are each of them 

 equal to the thousandth or the millionth part of a molecule of hydrogen, and 

 that they can traverse freely the interspaces of all ordinary molecules. But, as 

 we have seen, an equilibrium will establish itself between the agitation of the 

 ordinary molecules and those of the aether. In other words, the aether and the 



