84 SCIENCE PROGRESS 



fore, is left with eighty-six positive charges, so that it is no 

 longer radium, but a new element niton, or radium emana- 

 tion, of which the atomic number is 86. 



If, to take an imaginary case, mercury, with atomic number 

 80, expelled an a particle, it would lose two positive charges 

 and become the 78th element, platinum. If it lost a /3 particle 

 as well, it would lose two positive charges and one negative, 

 that is, one positive on the whole, and so it would retain 79 

 surplus positive charges on the nucleus and thus become gold. 

 But if the mercury lost one /9 particle only, and no a, the 

 surplus positive charges on the nucleus would become one 

 more, viz. 81, and thallium would be produced. If it lost two 

 negative charges, /3 particles, it would transmute itself to lead, 

 and so on for other combinations. 



But when we ask how this expulsion of particles is to be 

 brought about, we are face to face with the supreme difficulty 

 of the problem. In the radio-active elements the change is 

 spontaneous, and we are quite unable to accelerate or retard it, 

 or to influence it in any way. The particles are expelled, the 

 element is transmuted, and energy is liberated at a perfectly 

 constant rate. For the other elements we have to find some 

 projectile that is very tiny, and yet moving with an incredible 

 velocity, and with it bombard the atom and break up the 

 nucleus. The only projectiles that fulfil these conditions are 

 the a and /3 particles. 



Sir Ernest Rutherford has used them to bombard the 

 atoms of nitrogen, and has found that from a very small per- 

 centage of them atoms of hydrogen are produced. 



It remains to ask what light this theory of the structure of 

 the atom can throw on the vertical groups in the Periodic 

 Table, a subject amplified in the Presidential Address to the 

 British Association last year. We have already seen that the 

 positive charges, or protons, of the atom are concentrated on 

 the nucleus at the centre, and the electrons are around it. 

 Mathematical investigation has shown that this arrangement is 

 quite possible if the electrons are on a sphere and not too close 

 together. The mutual forces of repulsion between them will 

 prevent overcrowding, and Sir J. J. Thomson has proved that 

 when there are more than a certain number of electrons on the 

 sphere the force of attraction exerted by the positive nucleus 

 is not sufficient to maintain stable equilibrium, so that a new 

 arrangement is formed with different layers of electrons on 

 concentric spheres. The number of electrons on the outer 

 sphere depends upon the law of force between the nucleus and 

 the electrons and between the electrons themselves, and Sir 

 J. J. Thomson has shown that if this law of force is of a simple 

 type then the number of electrons that can exist in a layer is 



