May 13, 1922 



NATURE 



617 



that the helium nuclei are bound together with much 

 weaker forces than the H-components of the helium 

 nucleus itself. If the structure of the complex nucleus, 

 say, of oxygen is such that the a-particle may communi- 

 cate a considerable fraction of its momentum and 

 energy to a single component, we should expect such 

 disintegration to occur. It is also possible that charged 

 particles of mass about 2 or 3 may exist as secondary 

 building units of the complex nuclei of some elements, 



I but so far no definite evidence of their liberation has 



i. been obtained. 



} I have so far confined my remarks to the disintegrat- 

 ing effects of swift a-rays, but it is important to con- 

 sider whether the swift /8-rays or energetic y-rays from 

 radium are able to produce any effect. We have found 

 that neither ^- nor y-rays appear to have the power of 

 giving sufficient energy to a free H-atom in ordinary 

 hydrogen to detect it by the scintillation method, and 

 consequently still less should we expect these rays to 

 liberate a swift H-atom from a complex nucleus. It 

 is possible, however, that these agencies, and par- 

 ticularly y-rays of very short wave-length, may be 

 able to liberate an electron and lead in this way to a 

 change of its atomic number. Unless, however, the 

 resulting atom is unstable and breaks up with the 

 emission of a swift particle of the a-ray type, it will be 

 difficult to be certain of such transformations. It 

 should be noted that Slater has shown that a-rays are 

 able to excite some very penetrating rays in their 

 passage through ordinary matter. There is some 

 evidence that such high-frequency radiation can only 

 arise from the nuclei of atoms. If this be the case, it 

 may be possible that the a-rays in some cases lead to a 

 liberation of a /3-particle from the nucleus and a con- 

 sequent transformation. This effect, however, must 

 l)e on a very small scale. 



Many attempts have been made in the past to test 

 whether the ordinary atoms can be disintegrated by 

 special agencies. The late Sir William Ramsay, with 

 tlie characteristic instinct for choosing the best line of 

 attack, made a number of experiments on the effect 

 of the a-rays of radium on matter and concluded that 



he had obtained evidence of the production of neon 

 from water and the liberation of lithium from copper. 

 These conclusions have not been confirmed by subse- 

 quent investigators, and in the light of the experiments 

 described in this lecture it seems very doubtful whether 

 the amount of transformation, even if it occurred, 

 would be sufficient to be detected by ordinary chemical 

 methods such as were employed. 



Many instances have been recorded of the appear- 

 ance of helium in discharge tubes, and it has been sug- 

 gested that helium is a product of the transformation 

 of the electrodes by the action of the intense electric 

 discharges. The most notable experiments in this 

 direction have been made by Prof. Collie, but the 

 subsequent detailed investigations by Strutt did not 

 confirm his conclusions. It is exceedingly difficult to 

 prove that the appearance of helium is not due to its 

 occlusion in the electrodes and liberation by the intense 

 heating of the discharge. Sirrrilarly, many observa- 

 tions have been made of the steady liberation of 

 hydrogen from electrodes. Winchester, who examined 

 this effect in detail with thin aluminium electrodes, 

 found that hydrogen was released until the electrodes 

 were entirely dissipated. It is very difficult to believe 

 that this hydrogen is a product of the transformation 

 of aluminium when we remember the great energy of 

 the a-particle required to effect it. As in the case of 

 helium, it seems more probable that the hydrogen was 

 originally absorbed in the electrodes. 



While it is unsafe to be dogmatic on these points, 

 the general evidence indicates that the atoms as a 

 rule are such stable structures and the nuclei are 

 held together by such powerful forces that only a most 

 concentrated source of energy, such as the a-particle, is 

 likely to be effective in an attack on such well-protected 

 structures. Even when disintegration takes place, it 

 is on an exceedingly minute scale, and only a few 

 a-particles in a million are effective. If we had charged 

 atoms available of ten times the energy of the a- 

 particle from radium, we could probably penetrate the 

 nuclear structure of all atoms and occasionally effect 

 their disintegration. 



Organisation for Visual Instruction. 



By Dr. C. W. Kimmins, Chairman of 



A BULLETIN on the subject of visual instruction 

 recently received from the University of Wis- 

 consin is of great value as showing the remarkable 

 results which have been obtained by the collection and 

 distribution on an elaborate scale of material for the 

 development of this method of teaching. The fact that 

 in Wisconsin " the circuit plan of distribution " has 

 been in operation for six years " with increasingly 

 gratifying results," enables the authorities to speak 

 with great weight on the subject. The Bulletin deals 

 not only in considerable detail with every department 

 of the organisation, but also gives valuable suggestions 

 for overcoming difficulties such as the transport, storage, 

 and repairing of films and lantern slides. An idea of 

 the great importance attached to visual instruction by 

 the University may be obtained from the fact that a 

 room in the University extension building has been 

 fitted up and equipped with stereopticons, motion- 



NO. 2741, VOL. 109] 



the Cinema Education Committee. 



picture machines, and projection apparatus of many 

 of the leading makes. Teachers and committees are 

 invited to visit Madison at any time to inspect these 

 machines side by side and compare their respective 

 merits with one another. It is also possible at the same 

 time to become better acquainted with the visual 

 instruction plans and purposes of the university exten- 

 sion division as well as with the stock of material. 



In the British Isles the useful part which visual 

 instruction can play in educational methods has long 

 been recognised, as is evidenced by the action of the 

 more important local education authorities in providing 

 the schools with lanterns, lantern slides, pictures, 

 maps, diagrams, botanical and other specimens, and 

 a variety of other material for this purpose. The 

 Lantern SHde Committee of the London County Council 

 has done valuable work for many years in the prepara- 

 tion of a magnificent collection of lantern slides suitable 



