i 7 o SCIENCE PROGRESS 



traversed by the «-particles themselves. These results indicated 

 that the nuclei of the hydrogen and helium atoms must approach 

 during collision to within a distance of 1*3 x io~ 13 cm., or less 

 than the diameter usually accepted for the electron. It might 

 be that the mass of the nucleus was, like that of the electron, 

 electro-magnetic in character and was so much greater than 

 the mass of the electron because of this greater concentration 

 of the charge. On such a view the hydrogen nucleus might 

 be really the long-looked-for positive electron. 



Results obtained in other fields showed that the charge of 

 the nucleus was probably the " atomic number," which usually 

 is rather less than half the atomic weight. The atomic number 

 is the number of the element in the series when all the places 

 of the periodic table are arranged in sequence, that of hydrogen 

 being one, of helium, two, of lithium, three, and so on. Recent 

 results of Soddy and others, based upon the chemical work of 

 Fleck, showed that the value of the nuclear charge entirely 

 controlled the chemical properties of the atom. When a radio- 

 element expelled an a-particle the nuclear charge was reduced 

 by two units, and when it expelled a /3-particle the nuclear 

 charge was increased by one unit. The first produced a change 

 in chemical nature corresponding with a change of two places 

 in the periodic table in one direction and the second with a 

 change of one place in the opposite direction. But after one 

 a-particle and two /3-particles had been expelled, the product 

 was chemically indistinguishable from the parent. 



The constitution of the nucleus was a difficult problem 

 which might be left to the future generation of physicists, but 

 it was natural and indeed necessary to suppose in the case 

 of the radio-elements that the helium nucleus was one con- 

 stituent, and, possibly, the hydrogen nucleus was another. 

 But the theory of Dr. Bohr which attempted to reconcile the 

 older mechanical theor}' with the newer conception of energy 

 quanta undoubtedly had some relation to the experimental 

 facts and had achieved notable successes in accounting for the 

 wave-lengths of the luminous spectra of the simpler elements 

 and for the wave-lengths of the characteristic X-rays. 



Mr. Soddy, who followed, said he had found the nuclear 

 atom of great help as a guide in following the sequence of 

 radioactive changes. It was clear that particles moving like 

 the a-particles, at what might be termed ultra-material 



