September 23, 1920] 



NATURE 



117 



been extended over more and more branches of 

 physics. 



We shall not deal here with these spectroscopic 

 questions, but it will be convenient to have a 

 definite idea of the scale on which the atom is 

 built. The proportions of its parts vary over so 

 wide a range that no drawing can possibly repre- 

 sent them. We shall be dealing with magnitudes 

 as small as lo-"* cm., and as these large negative 

 indices convey little definite to the mind, it will 

 be convenient in describing the atom to raise the 

 whole scale by lo^^. On this scale i cm. would 

 become a length about two-thirds of the distance 

 from the earth to .the sun. The outermost elec- 

 trons of the atom would be about a kilometre from 

 the nucleus, and for the heavier elements the 

 innermost, perhaps three in number, would be 

 roughly 10 metres from it. As to the nucleus 

 itself, there is definite evidence that it is less ihn.n 

 30 cm. in radius, and, in some cases at any rate, 

 that it is greater than 2 cm. Other physical 

 quantities which we shall require are of a slightly 

 more hypothetical nature. On the theory that all 

 mass is electromagnetic, we can calculate the 

 radius of an electron, since we know its charge 

 and mass. This radius comes to about 2 cm. 

 (actually i-88x iq-" cm.). By a similar calcula- 

 tion the hydrogen nucleus has a radius of about 

 one-hundredth of a millimetre. The same argu- 

 ment would make the radii of heavier nuclei about 

 as small as this, but would not be justifiable, 

 because, as we shall see, there is clear evidence 

 that these nuclei owe their large mass to their 

 being composite structures built up from hydrogen 

 nuclei and electrons. From the extraordinary 

 smallness of the quantities we have considered, it 

 is not surprising that experiments with the 

 nucleus should be very diflicult. We now turn 

 to these experiments. 



When first describing the nucleus, in the first 

 article, we saw that when an a-particle goes 

 near it, it describes a calculable orbit, so that vve 

 can find a distance of closest approach between 

 the two. As the predicted law of scattering was 

 borne out by experiment, we can conclude that 

 the law of force at this distance of closest ap- 

 proach (3x10"** cm. for gold, above called 

 30 cm.) is still the inverse square law. There- 

 fore the sum of the radii of the a-particle and the 

 nucleus must be less than this. But a different 

 case has been studied in great detail by Ruther- 

 ford. Where the o-particles are passing through 

 hydrogen, the repulsive forces between the nuclei 

 are much weaker than for other substances, 

 because the atomic number of hydrogen is only i, 

 and so the nuclei can get much closer together. 

 Hut there is also another difference, for the hydro- 

 gen nucleus has mass only a quarter of that of 

 the a-particlc. Consequently, if there is a 

 "straight on" collision the hydrogen is shot 

 forward at a speed higher than that of the 

 colliding o-particle, and the latter follows on 

 behind with reduced speed. This new type of 

 particle, the H-particle, then proceeds to traverse 



NO. 2656, VOL. 106] 



any matter in its path, losing speed in the same 

 sort of way as does an o-particle, and it can be 

 calculated that, whereas the fastest o-particles 

 go through only 7 cm. of air, the fastest of these 

 H-particles should go through 28 cm. (Fig. 2). 

 Experiment bore out this prediction, but when the 

 calculation was pushed further so as to show how 

 many should go various distances from 28 cm. 

 downwards, a very wide difference was found 

 between theory and experiment. This indicates 

 that there is something wrong with the theory — 

 in fact, that the approach between the nuclei is 

 so close that the ordinary law of repulsion fails. 

 This ordinary law would give 2 x io~'* cm. as the 

 distance of closest approach, and we therefore 

 conclude that the sum of the radii of the hydrogen 

 and helium nuclei is greater than this. Now the 



10 



13 16 19 22 25 



/ienge of HParticles In Air in Cms. 



Fig. a. — Froir Sir Ernest Rulherford's paper "The Cotli&ion of 

 a-ParticIes with Light Atomn" (Phti. Mag., xxxvii., p. 540). 

 The wide difference between the "obscrvecl" and "calculated" 

 curve* shows that the assumed law of force between a-particle 

 and H-nudeus is wrong ; in fact, that a lower limit haa been 

 obtained to the size of the nucleus. 



radius of the electron is about 2 x io~'' cm., and 

 the coincidence of these two numbers tends to 

 support the view that there are electrons in the 

 nucleus of the helium atom. 



Turning now to the constitution of the nucleus, 

 the first piece of information is provided by radio- 

 activity. In successive radio-active transforma- 

 tions a- and /3-particles are projected from the 

 nucleus. Therefore the latter must contain both — 

 that is, helium nuclei and electrons. Next we 

 have the evidence from Rutherford's transmuta- 

 tion of elements. The experiments in which this 

 occurred consisted in the bombardment of various 

 elements by o-particles. When nitrogen was 

 tried, he observed that a certain number of H-par- 

 ticles was obtained. As no hydrogen was present, 

 we are compelled to conclude that these particles 

 were knocked off the nuclei of the nitrogen atoms; 

 so we can be confident that these nuclei contain 

 hydrogen nuclei. In recent work Rutherford has got 

 even further, for he has found another entirely new 

 type of particle which is sometimes knocked off 

 nitrogen and oxygen. These particles have atomic 

 mass 3 and number 2, so that if they were 

 obtained in bulk they would be helium of density 

 25 per cent, less than ordinary helium. It is at 

 present unknown what happens to the remainder 

 of the nucleus after these losses. .\ point of great 



