412 



Supplement to "Nature" September 15, 1923 



Ijetwcen the units of positive and negative electricity 

 is emphasised. In fact, as we shall see later, the atoms 

 are quite unsymmetrical structures with regard to the 

 positive and negative units contained in them, and 

 indeed it seems certain that if there were not this 

 difference in mass Iwtween the two units, matter, as 

 we know it, could not exist. 



It is natural to inquire what explanation can be 

 given of this striking difference in mass of the two 

 units. I think all scientific men are convinced that the 

 small mass of the negative electron is to be associated 

 entirely with the energy of its electrical structure, 

 so that the electron may be regarded as a disembodied 

 atom of negative electricity. We know that an 

 electron in motion, in addition to possessing an electric 

 field, also generates a magnetic field around it, and 

 energy in the electromagnetic form is stored in the 

 medium and moves with it. This gives the electron 

 an apparent or electrical mass, which, while nearly 

 constant for slow speeds, increases rapidly as its velocity 

 approaches that of light. This increase of mass is in 

 good accord with calculation, whether based on the 

 ordinary electrical theory or on the theory of relativity. 

 Now we know that the hydrogen atom is the lightest 

 of all atoms, and is presumably the simplest in structure, 

 and that the charged hydrogen atom, which we shall 

 see is to be regarded as the hydrogen nucleus, carries 

 a unit positive charge. It is thus natural to suppose 

 that the hydrogen nucleus is the atom of positive 

 electricity, or positive electron, analogous to the 

 negative electron, but differing from it in mass. 

 Electrical theory shows that the mass of a given charge 

 of electricity increases with the concentration, and the 

 greater mass of the hydrogen nucleus would be 

 accounted for if its size were much smaller than that 

 of the electron. Such a conclusion is supported by 

 evidence obtained from the study of the close collisions 

 of a-particles with hydrogen nuclei. It is found that 

 the hydrogen nucleus must be of minute size, of radius 

 less than the electron, which is usually supposed to be 

 about 10""^' cm.; also the experimental evidence is 

 not inconsistent with the -view that the hydrogen 

 nucleus may actually be much smaller than the electron. 

 While the greater mass of the positive atom of electricity 

 may be explained in this way, we are still left with the 

 enigma why the two units of electricity should differ 

 so markedly in this respect. In the present state of 

 our knowledge it does not seem possible to push this 

 inquiry further, or to discuss the problem of the 

 relation of these two units. 



We shall see that there is the strongest evidence 

 that the atoms of matter are built up of these two 

 electrical units, namely the electron and the hydrogen 

 nucleus or proton, as it is usually called when it forms 

 part of the structure of any atom. It is probable 

 that these two are the fundamental and indivisible 

 units which build up our universe, but we may reserve 

 in our mind the possibility that further inquiry may 

 some day show that these units are complex, and 

 divisible into even more fundamental entities. On the 

 views we have outlined, the mass of the atom is the 

 sum of the electrical masses of the individual charged 

 units composing its structure, and there is no need to 

 assume that any other kind of mass exists. At the 

 same time, it is to be borne in mind that the actual 



mass of an atom may f)c somewhat less than the st; 

 of the masses of c:omponcnt positive and n< 

 electrons when in the free state. On account 

 very close proximity of the charged units in the nu« it 

 of an atom, and the consequent disturl>ance of tl 

 electric and magnetic fields surrounding them, such 

 decrease of mass is to be anticipated on gen« ml iiw 

 retical grounds. 



We must now look hack again to the eain. • 

 of the present epoch in order to trace the develf- 

 of our ideas on the detailed structure of the 

 That electrons as such were important const: 

 was clear by 1900, but little real pr' 

 until the part played by the positi. 

 made clear. New light was thrown on thi> 

 examining the deviation of a-particles v 

 passed through the atoms of matter. It wa.s 

 that occasionally a swift o-particle was deflects > 

 its rectilinear path through more than a right 

 by an encounter with a single atom. In such a C' 

 the laws of dynamics ordinarily apply, and the r^ 

 between the velocities of the colliding atoms ikk 

 and after collision are exactly the same as if the tv 

 colliding particles are regarded as perfectly > ' 

 spheres of minute dimensions. It must, howe\ 

 borne in mind that in these atomic collisions t; 

 no question of mechanical impacts such as we oi 

 with ordinary matter. The reaction between the tvr- 

 particles occurs through the intermediary of the pow( 

 ful electric fields that surround them. Beautif 

 photographs illustrating the accuracy of these laws > 

 collision between an a-particle and an atom have be« 

 obtained by Messrs. Wilson, Blackett, and other 

 while Mr. Wilson has recently obtained many strikii 

 illustrations of collisions between two electrons. R 

 membering the great kinetic energy of the a-partirl. 

 its deflexion through a large angle in a single ; • 

 encounter shows clearly that ver}' intense defli 

 forces exist inside the atom. It seemed clear tli. 

 electric -fields of the required magnitude could i 

 obtained only if the main charge of the atom wc: 

 concentrated in a minute nucleus. From this aro- 

 the conception of the nuclear atom, now so well known, 

 in which the heart of the atom is supposed to consist 

 of a minute but massive nucleus, carrj'ing a positi\e 

 charge of electricity, and surrounded at a distance by 

 the requisite number cf electrons to form a neutral 

 atom. 



A detailed study of the scattering of a-particles ; 

 different angles, by Geiger and Marsden, showed th. 

 the results were in close accord \nth this theory, ai 

 that the intense electric forces near the nucleus varit < ; 

 according to the ordinary- inverse square law. 1; 

 addition, the experiments allowed us to fix an upp' 

 limit for the dimensions of the nucleus. For a hea\ 

 atom like that of gold the radius of the nucleus, 

 supposed to be spherical, was less than one-thousandt 

 of the radius of the complete atom surrounded by i' 

 electrons, and certainly less than 4x10-^2 ^-n^ \ 

 the atoms were found to show this nuclear structur' 

 and an approximate estimate was made of the nuclea: 

 charge of different atoms. This type of nuclear atom, 

 based on direct experimental evidence, possesses son 

 very simple properties. It is obvious that the numbi 

 of units of resultant positive charge in the nucleus 



