ATOM. 



media, the amalleflt aensible portion of which contains millions of millions of 

 The constancy and uniformity of the properties of the gaseous 

 medium * the direct result of the inconceivable irregularity of the motion of 

 agitation of it molecule*. Any cause which could introduce regularity into the 

 motion of agitation, and marshal the molecules into order and method in tin ir 

 vrolutiuna, might olut-k or even reverse that tendency to diffusion of matter, 

 motion, and energy, which is one of the most invariable phenomena of nature, 

 nnd to v hich Thomson has given the name of the dissipation of energy. 



Thus, when a sound-wave is passing through a mass of air, this motion 

 is of a certain definite type, and if left to itself the whole motion is passed on 

 to other masses of air, and the sound-wave passes on, leaving the air behind 

 it at rest Heat, on the other hand, never passes out of a hot body except 

 to enter a colder body, so that the energy of sound-waves, or any other form 

 of energy which is propagated so as to pass wholly out of one portion of 

 the medium and into another, cannot be called heat. 



\V-- have now to turn our attention to a class of molecular motions, which 

 are as remarkable for their regularity as the motion of agitation is for its 

 irregularity. 



It has been found, by means of the spectroscope, that the light emitted 

 l)\- incandescent substances is different according to their state of condensation. 

 When they are in an extremely rarefied condition the spectrum of their light 

 consists of a set of sharply-defined bright lines. As the substance approaches 

 a denser condition the spectrum tends to become continuous, either by the 

 lines becoming broader and less defined, or by new lines and bands appearing 

 between them, till the spectrum at length loses all its characteristics and 

 becomes identical with that of solid bodies when raised to the same tem- 

 perature. 



I Luce the vibrating systems, which are the source of the emitted light, 

 must be vibrating in a different manner in these two cases. When the spectrum 

 consists of a number of bright lines, the motion of the system must be com- 

 pounded of a corresponding number of types of harmonic vibration. 



In order that a bright line may be sharply defined, the vibratory motion 

 which produces it must be kept up in a perfectly regular manner for some 

 hundreds or thousands of vibrations. If the motion of each of the vibrating 

 bodies is kept up only during a small number of vibrations, then, however 

 regular may be the vibrations of each body while it lasts, the resultant dis- 



