1919] on Ether and Matter 465 



cultj) that its energy is absorbed till a quantum has been accumulated, 

 and then a ^ raj or excessively rapid electron is emitted. 



Remarks on the Quantum. 



In my view it should not be thought that energy exists in numerical 

 bundles, or quanta ; the discontinuity is not really in energy, but in 

 the atom. Atomic properties are essentially numerical and discon- 

 tinuous, and we ought not to be surprised at an equilibrium which 

 needs a specific amount of energy to upset it. The energy must be 

 supplied by the disturbing impulse : but in the case of ultra-violet or 

 X-ray radiation, the energy can only be attributed to the disturbing 

 impulse on the principle of resonant or syntonic accumulation ; for 

 its intensity does not matter. Nor ought it to matter so long as 

 the tuned impulse is repeated often enough — a repetition for which an 

 extremely minute fraction of a second is ample. What matters is 

 not the brightness or energy of the incident radiation therefore, 

 but its frequency. On the other hand, a (i projectile cannot effect a 

 real disturbance unless it possesses a minimum quantum of energy ; 

 for in that case there is no accumulation. 



The quantum, considered merely as a finite store of energy, is 

 susceptible of exceedingly elementary illustration. Here is a case 

 of stable equilibrium (a simple pendulum, or a round- bottomed 

 flask loaded so as to oscillate stably) which responds to the slightest 

 touch and returns to equilibrium. There is no quantum about that. 

 But here is another case of stable equilibrium (a brick or block or 

 pillar standing on end) which takes no notice of any but a finite 

 force, and which requires a finite amount of energy to upset it — viz. 

 its weight multiplied by the elevation of its centre of gravity as it 

 revolves round its lower edge : this being also the amount of energy 

 emitted when it falls. Or there may be a union of the two kinds of 

 equilibrium. This rounded rocking flask, for instance, or a rocking- 

 horse, may accumulate oscillations till the energy reaches a sort of 

 quantum, when it upsets and breaks or causes an accident. This 

 last is the kind of stable equilibrium which we meet with in an atom. 



A flying particle below a certain limit of energy can alter the 

 excentricity of an orbit, and may thus excite some simple radiations 

 which continue till the orbit becomes circular again ; but a synchronous 

 X-ray disturbance, however intrinsically feeble, may precipitate a 

 catastrophe ; and simple facts of this kind seem to be in the main 

 responsible for the general notion of quanta of energy. The really 

 remarkable thing about a quantum, the thing which makes it so 

 essentially worthy of attention, is the fact that it is a universal 

 constant : the same amount of energy is found associated with every 

 kind of matter — the same, or differing only by simple multiplication. 

 Hence the notion, at one time put forward, that energy itself might 

 be atomic, and exist in indivisible packets, like cartridges. 



