September 25, 1919] 



NATURE 



^3 



might exhibit chemical properties, perliaps, under 

 enormous pressure. The heavier atoms contain the 

 most particles, and must have the most complicated 

 structure. There is every grade, from the simplest, 

 hydrogen, with one electron, to the most comple.i, 

 uranium, with ninety-two. There is room for ninety- 

 two elements in the series, and no more. All these 

 are actually known except five or six. There are only 

 these few unfilled gaps in the chemical series of 

 elements as thus planned. 



liaAio-aclivxXy . 



A complicated atom has a certain amount of in- 

 stability, and may fall down occasionally into the 

 next simple grouping, flinging away one or more of 

 its units. When this happens there is a sort of 

 atomic cataclysm or explosion ; a projectile and some 

 quanta of energy are emitted. This is the pheno- 

 menon of radio-activity. Uranium after three (or 

 possibly four) such eruptions becomes the element 

 three (or four) steps down the series, viz. radium. 

 Radium after five more explosions becomes apparently 

 the well-known and stable element lead, or at least 

 something chemically indistinguishable from it, though 

 perhaps of slightly different atomic weight, — what has 

 recently been called an "isotope" of lead. That is 

 the kind of statement that without too much rash- 

 ness can be cautiously and tentatively made. 



At every serious cataclysm an a-particle or atom 

 of helium is emitted from the nucleus, accompanied 

 bv a ;S-particle or negative corpuscle from somewhere, 

 usually from the planetary system. A sympathetic 

 aethereal gush of y-rays accompanies the eruption. A 

 definite unit of energy — a quantum or a simple 

 multiple of it — is emitted at each explosion; and the 

 remaining electrons then settle down into their new 

 orbits, the element changing in character and chemical 

 properties accordingly. 



A catastrophe of this kind can be produced by a 

 sufficiently rapid projectile, an a- or /3-particle shot 

 off, say, bv radium ; and a minor catastrophe or 

 emission of a ^-particle can also be produced by the 

 accumulated energy of properly attuned X-rays. 

 When art X-ray or ray of ultra-violet light agrees in 

 frequencv with the orbital frequency of an electron, 

 we can suppose (not without a little difficulty) that 

 its energy is absorbed until a quantum has been 

 accumulated, and then a /3-ray 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 discon- 

 tinuity is not really in energy, but in the atom. 

 Atomic properties are essentially numerical and dis- 

 continuous, 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 /3-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 illus- 

 tration. 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 



NO. 2604, VOL. 104] 



and returns to equilibrium. There is no quantum 

 about that. But here is another case of stable equili- 

 brium (a brick or block or pillar standing on end) 

 which takes no notice of any but a finite force, and 

 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 equili- 

 brium. This rounded rocking flask, for instance, or 

 a rocking-horse, may accumulate oscillations until 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 eccentricity of an orbit, and may thus excite 

 some simple radiations which continue until the orbit 

 becomes circular again ; but a synchronous X-rav dis- 

 turbance, 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 asso- 

 ciated with every kind of matter — the same, or differ- 

 ing 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. 



Hypothetical Structure of Atoms. 



The real facts concerning the quantum, which are 

 the result of observation, suggest, when interpreted 

 properly, that there are stable electronic orbits in an 

 atom, and that these follow a regular law of succes- 

 sion, analogous perhaps to Bode's law of planetary 

 distances in the solar system. Spectroscopic evidence 

 —the so-called Balmer's series of lines — strongly bears 

 out this idea. For there is what is called K radia- 

 tion, of highest frequency, apparently due to per- 

 turbations of the innermost, the most rapid, ring ; 

 L radiation of lower frequency from the next outer 

 ring ; M radiation from a ring outside this ; and 

 recently there is talk of a J radiation of extra high 

 frequency from a ring still closer to the nucleus — 

 perhaps quite close to it, part of it perhaps — and, 

 anyway, well within the K ring. 



The frequencies adapted to bring about an atomic 

 catastrophe, or which are emitted during perturba- 

 tions, are usually high up in the series of X-ray series 

 of vibrations, far above visible light. I assume that 

 these frequencies correspond with the freauencv of 

 orbital revolution, and that the inverse-square law 

 holds good. The more massive the nucleus, the 

 greater must be the frequency of orbital revolution 

 at a given distance, in accordance with Kepler's third 

 law. The square of the frequency multiplied bv the 

 cube of the radius of the orbit will be constant for 

 all the orbits of all the atoms of any given substance, 

 and will give the attracting force of the nuclear centre 

 for that substance. 



In other words, this product (or, what comes to the 

 same thing, the radius multiplied by the square of 

 the speed) will correspond with the number of un- 

 neutralised positive charges which go to make up the 

 nucleus. It will give, in fact, the number of the 

 element in the Mendcleeff series. The K radiation 

 frequency from uranium, therefore, must be excep- 

 tionally rapid, because the nucleus is so strong. For 

 hydrogen, the nucleus of which is only i/q2nd of 

 that of uranium, the orbital frequency might be com- 

 paratively slow, not higher than the ultra-violet ; while 

 the L radiation from hydrogen, it is now thought, 



