146 CONCEPTION OF THE CHEMICAL ELEMENT 



constitution of the nucleus, and the integral variation 

 of this charge from i to 92 gives the successive 

 places of the periodic table. Expulsion of two 8- 

 and one a-particle in any order gives an isotope of 

 the original element with atomic weight four units 

 less. Isobaric isotopes resulting in branch changes 

 differ only in the internal structure and stability of 

 the nucleus. The atomic mass is the only nuclear 

 property known before the discovery of radioactivity, 

 and, except as regards this, the whole of physics and ' 

 chemistry up to the close of the nineteenth century 

 had not penetrated beyond the outer electronic shell 

 of the atom. Even now, mass and radioactivity 

 remain the sole nuclear properties known. 



CONCLUSION. 



Nemesis, swift and complete, has indeed over- 

 taken the most conservative conception in the most 

 conservative of sciences. The first phase robbed the 

 chemical element of its time-honoured title to be 

 considered the ultimate unchanging constituent of 

 matter ; but since its changes were spontaneous and 

 beyond the power of science to imitate or influence 

 to the slightest degree, the original conception of 

 Boyle, the practical definition of the element as the 

 limit to which the analysis of matter had been pushed, 

 was left essentially almost unchanged. 



The century that began with Dalton and ended 

 with the discoveries of Becquerel and the Curies 

 took the existing practical conception of the chemical 

 element and theorised it almost out of recognition. 

 The element was first atomised, and then the atom 

 was made the central conception of the theory of the 

 ultimate constitution of matter, on which modern 

 chemistry has been reared, and from which its 

 marvellous achievements, both practical and theo- 

 retical, have mainly sprung. The atom and the 



