MECHANICS AND OPTICS. 233 



Nevertheless, the effects of this wave of acceleration, 

 and more particularly the corresponding loss of energy, 

 are negligible in the majority of cases — that is to say, 

 not only in the ordinary Mechanics and in the motions 

 of the celestial bodies, but even in the case of the radium 

 rays, where the velocity, but not the acceleration, is 

 very great. We may then content ourselves with the 

 application of the laws of Mechanics, stating that the 

 force is equal to the product of the acceleration and 

 the mass, this mass, however, varying with the velocity 

 according to the laws set forth above. The motion is 

 then said to be quasi-stationary. 



It is not so in all the cases where the acceleration is 

 great, the chief of which are as follows, (i.) In incan- 

 descent gases certain electrons take on an oscillatory 

 motion of very high frequency ; the displacements are 

 very small, the velocities finite, and the accelerations 

 very great ; the energy is then communicated to the 

 ether, and it is for this reason that these gases radiate 

 light of the same periodicity as the oscillations of the 

 electron. (2.) Inversely, when a gas receives light, 

 these same electrons are set in motion with violent 

 accelerations, and they absorb light. (3.) In Hertz's 

 excitator, the electrons which circulate in the metallic 

 mass undergo a sudden acceleration at the moment of 

 the discharge, and then take on an oscillatory motion 

 of high frequency. It follows that a part of the energy 

 is radiated in the form of Hertzian waves. (4.) In an 

 incandescent metal, the electrons enclosed in the metal 

 are animated with great velocities. On arriving at the 

 surface of the metal, which they cannot cross, they are 

 deflected, and so undergo a considerable acceleration, 

 and it is for this reason that the metal emits licrht. 



