36 The Realm of Nature CHAP. 



As long as there is any kinetic energy associated with a 

 portion of matter the particles will be in motion. The 

 amplitude of the oscillations in solids is very slight, not 

 sufficient to overcome the resistance of cohesion ( 39). 

 However large a body may be, its particles will in time 

 come to oscillate at the same rate throughout if not inter- 

 fered with, any more quickly-moving particles passing on 

 some of their energy to their more slowly -moving neigh- 

 bours. The process of passing on and equalising the rate 

 of molecular vibration is called conduction, and takes place, 

 although more slowly, in liquids and gases as well as in 

 solids. 



60. Radiant Energy. As the vibrations of bodies, as 

 a whole, set up waves of various length in air which may 

 travel to a distance, and some of which are capable of 

 impressing the ear, so the invisible vibration of the particles 

 of bodies sets up waves of radiant energy which travel to 

 a distance, and some of which impress the senses. The 

 quiverings of particles are very complex, and the particles 

 of each kind of matter seem to quiver and oscillate in a 

 way of their own, setting up waves which, although ex- 

 cessively minute, are far more complex than those of sound. 

 There is much difficulty in understanding how the waves of 

 radiant energy travel, and it is assumed that a very remark- 

 able kind of matter called the Ether fills all space, and 

 penetrates freely between the particles of ordinary matter. 

 It is so fine that it offers no perceptible resistance to the 

 movement of the planets through it, or to the movements 

 of the particles of matter ; but it is so elastic that it passes 

 on the smallest and swiftest undulations. The undulations 

 travel in straight lines through the ether at the rate of 

 nearly 186,000 miles per second, and all amplitudes of 

 these undulations travel at the same rate, about a million 

 times as fast as the waves of sound in air. 



6 1. Reflection and Refraction. When the waves of 

 radiant energy reach a surface through which they cannot 

 pass, they are turned into a new path, either directly back- 

 ward or at a definite angle to their former direction. 

 Sound-waves meeting an obstacle are reflected in the same 



