6i6 



NATURE 



[May 13, 1922 



If we suppose that the H-sateUite is describing an 

 orbit round the central nucleus, the direction of 

 escape will depend on the relative position of the 

 a-particle and nucleus at the moment of the close 

 collision with the satellite. In the collision shown in 



Fig. 3. 



Fig. 3, A, for example, the H-atom will escape in the 

 forward direction of the a-particle ; in the colHsion 

 (Fig. 3, B) the H-atom will describe an orbit round 

 the nucleus and escape in the backward direction. 

 The velocity imparted to the residual nucleus in the 

 forward direction is much greater in the latter than 

 in the former case. Such a view assumes that the 

 forces between the positively charged satellite and 

 nucleus are attractive instead of repulsive very close 

 to the nucleus. This change of sign of the forces seems 

 very probable at short distances from the nucleus, for 

 otherwise it is difficult to understand how a positively 

 charged complex nucleus can hold together. 



Another consequence of some interest follows from 

 the possibility of releasing H-atoms from light elements. 

 It is generally supposed, although it is very difficult 

 to obtain direct proof, that the helium nucleus is 

 composed of four hydrogen nuclei and two electrons. 

 In this combination there is a loss of mass, and this 

 is ascribed to the very close combination of the struc- 

 tural units. On modern views of the relation between 

 mass and energy, it follows that the energy liberated 

 in the formation of a helium nucleus is more than 

 three times the energy of the swiftest a-particle from 

 radium. We should consequently not expect to be 

 able to break up a helium nucleus with an ordinary 

 a-particle, and this is in agreement with experiment 

 so far as it has gone. In fact, the helium nucleus 

 would appear to be the most stable of all nuclei. 



Since, however, in the case of nitrogen, for example, 

 we are able tO release an H-atom by means of a slow 

 a-particle, it seems clear that the H-satellite is not 

 bound nearly so closely in the nitrogen nucleus as in 

 the case of the helium nucleus. The change of mass 

 due to the emission of energy in binding the H-satellite 

 should consequently be much less than in the case of 

 helium. The mass of the satellite should not differ 

 much from the free H-nucleus of mass 1*0077 i'^ terms 

 of = 16. 



If it be supposed that the nitrogen nucleus, for 

 example, is made up of three helium nuclei of total 

 mass 12 and two hydrogen nuclei, the mass of the 

 nitrogen atom should not be exactly 14-00, but more 

 nearly 14-01. In the case of the light elements, it 

 appears probable that the effective mass of the protons 

 composing the nuclei will vary in different atoms from 

 about 1-007 to I -000, depending on the closeness of the 

 combination. Consequently, we should expect that 

 the whole number rule found by Aston, which appears 



NO. 2741, VOL. 109] 



to hold for atomic masses to about i in 1000, would 

 be departed from if measurements could be made with 

 still further accuracy. 



The next question which arises is whether any other 

 particles beside that of hydrogen can be released by 

 a-ray bombardment. Some time ago, I found that 

 when radium-C was used as a source a small number 

 of bright scintillations were observed, which had a 

 maximum range in air of about 9 cm. It was natural 

 at first to suppose that these were due to a new type 

 of a-rays from the radioactive source. The effect, 

 however, of aluminium screens in reducing the range 

 of these particles led me at first to believe they were 

 generated in the volume of the gases used, namely, 

 nitrogen and oxygen. By comparing the bending of 

 these rays in a magnetic field with that of H-particles 

 from hydrogen, I concluded that they must be atoms 

 of mass about 3 carrying two positive charges. Later 

 experiments have brought home to me the untrust- 

 worthiness of this method of fixing the source of the 

 radiation on account of the marked variation in 

 thickness of films of metal foil. Using a more direct 

 and simpler method, I have recently convinced myself 

 that, at any rate in the case of oxygen, the particles 

 have their origin in the radioactive source and not 

 in the volume of the surrounding gas. Under such 

 conditions, the comparative method of estimating the 

 mass of the particles is no longer trustworthy. While 

 a large amount of experiment will be required to fix 

 definitely the nature of the radiation, the general 

 evidence indicates that it consists of particles of mass 

 4, which are projected from the source and represent 

 a new mode of transformation of radium-C. 



By the methods outlined we can hope to detect only 

 particles which travel a distance greater than the 

 primary a-particles. If, however, a disintegration of 

 an element should occur in which a massive particle is 

 liberated, it is quite probable that the latter may have 

 a range shorter than the a-particle. To examine cases 

 of this kind, we can utilise the beautiful method de- 

 veloped by Mr. C. T. R. Wilson for showing the trails 

 of ionising particles. Some experiments of this kind 

 by a modified method have been made by Mr. Shimizu 

 in the Cavendish Laboratory. A number of photo- 

 graphs showing well-marked tracks near the end of the 

 range of the particle have been obtained. Until, how- 

 ever, these photographs are accurately measured up and 

 compared with one another, it is difficult to be certain 

 whether or not these branching tracks can be explained 

 by collisions of the a-particle with the nuclei of nitrogen 

 or oxygen. It seems clear, however, that the nuclei 

 involved can travel considerable distances through the 

 gas before being absorbed. If a large number of photo- 

 graphs be taken, it should be possible to settle de- 

 finitely whether any collisions involving the disrup- 

 tion of atoms occur and to determine the probability 

 of their occurrence. This direct method of attack of 

 the problem, whilst laborious, should give very valuable 

 information on this point. 



It appears not improbable that the a-particle may 

 occasionally be able to disrupt a helium atom from a 

 complex nucleus like carbon or oxygen, which are be- 

 lieved to be composed of three and four helium nuclei 

 respectively. The fact that the mass of these atoms is 

 nearly an integral multiple of the helium atom suggests 



