202 



DISCOVERY 



to describe. The apparatus he used is shown in Fig. i. 

 He found that if a thin piece of magnesium foil, equiva- 

 lent in stopping power to 7 cm. or more of air, were 

 interposed between the source of a-particles and the 

 zinc sulphide screen, on which they could be detected, 

 the screen was quite dark. This is, of course, an 

 old result, and what was to be expected. But the 

 new result was this, that when aluminium foil was 

 substituted for the magnesium many scintillations 

 apf)eared on the screen, and further examination 

 showed that the particles producing these scintillations 

 were not a-particles, but particles that could travel as 

 far as 90 cm. in air before they were completeh' 

 stopped. They were proved to be hydrogen nuclei 

 which must have been broken off from those aluminium 

 nuclei which had had a head-on collision with the 



•It is probably the latter which are ejected when the 

 a-particle strikes. The elements whose atomic weights 

 are less than 32 but not expressible by the above for- 

 mulae, do not appear to be disintegrated by a-particles, 

 and all elements with atomic weights greater than 

 32 are similarly inert. In the case of aluminium, which 

 shows the phenomenon best of aU, it has been sho\vTi 

 that the energy of the liberated hydrogen is greater 

 than that of the liberating a-particle ; which means 

 that some of the internal energy of the atom has been 

 tapped. This liberation of internal energy occurs, as 

 has been mentioned, spontaneouslv in radio-active 

 bodies, but it is important to have direct proof of it 

 from those elements artificiaUy disintegrated. 



Let me say this again in different words. Of the 

 eighty-seven known chemical elements, six at present 



M 



la a glass tube which may be evacuated is inserted a disc D containing the body which expels a-particles. It can be brought nearer 5 by the screw R. 

 S is the screen which emits light when stmck by the particles from D. The flashes are observed by looking through the microscope M focused on it. 

 .4 is a movable screen placed in front of D so as to be bombarded by the a-particles . 



o-particles directed against them. The number of 

 aluminium nuclei struck in this way is relatively 

 extremely small. Rutherford has calculated that only 

 about two a-particles in every million get near enough 

 to the nucleus to be able to break off a piece of hvdrogen, 

 although each passes through about a hundred 

 thousand atoms of aluminium before it is stopped ; 

 that is to say, there are only two reaUy successful hits 

 in a hundred thousand million shots. 



Elements which may be Transmuted 



Of the elements experimented on in this way, six 

 only have been artificially disintegrated. They are 

 boron, nitrogen, fluorine, sodium, aluminium, and 

 phosphorus. These bodies have atomic weights 

 expressible by the formulfe 4w x 3 or 4M x 2 where 

 n is an integer and, since 4 is the atomic weight of 

 helium, their nuclei may be regarded as made up of 

 helium nuclei with 3 or 2 attendant hydrogen nuclei. 



have been artificially disintegrated and four of these 

 are very common elements. The disintegration or 

 transmutation appears to be quite genuine. In all 

 si.x cases the piece knocked out is the element hydrogen, 

 and the piece that remains becomes a new element. 

 In some cases there is energy " tapped " from the 

 source of internal energy of the atom. But the only 

 means of procuring this type of transmutation arti- 

 ficially is the high-speed a-particles emitted by radio- 

 elements, and these are and alwaj's wiU be among the 

 verj- rarest of substances. So that, although all these 

 processes are of great interest to the theoretical scien- 

 tist, there has been just enough done to encourage the 

 exploiter of scientific knowledge, the man whom people 

 describe so glibly as about to tap the hidden forces of 

 the atom, if not to hope, at least not to despair. 



Conclusion 



To sum up in a few sentences. The dream of the 



