ISOTOPES 103 



parent. It is chemically identical with it. Elements 

 which so occupy the same place in the Periodic 

 Table and are absolutely identical in all their 

 chemical properties are called isotopes. The recog- 

 nition of such isotopes is fundamentally new, and 

 cuts more deeply into old-established ideas of the 

 nature of matter than even the surprising discoveries 

 of the genesis of one element out of another. 



The present theory of atomic structure is due to 

 Rutherford, and is based on experiments on the 

 course followed by an a-particle when it ploughs its 

 way through the atoms of matter. These experi- 

 ments have shown that the atom consists of a central 

 nucleus, possessing all but a negligible part of the 

 atomic mass but occupying only an exceedingly 

 minute fraction of the atomic volume. The nucleus 

 contains a preponderance of positive charges and is 

 surrounded by an equivalent number of separate 

 negative electrons, revolving in a system around it. 

 This theory lent itself at once to the interpretation 

 of the new developments here referred to, and both 

 together, along with very important work by the late 

 H. G. J. Moseley on the wave-lengths of the X-ray 

 spectra of the elements, have furnished the key to 

 the deciphering of the Periodic Law. It is melan- 

 choly to record that Moseley fell at Suvla Bay, aged 

 only twenty-eight. 



Prior knowledge of the atoms of matter has been 

 superficial in the literal sense confined entirely to 

 the outermost shell of the atom. We have now 

 penetrated to the interior and find, first, an inner 

 shell, wherein X-rays take their origin, and, secondly, 

 still further to the nucleus, the sanctum sanctorum of 

 the atom, revealed only by radioactivity and alone 

 concerned in this phenomenon. The same outer 

 and inner shells that is, the same kind of atom 

 to the older knowledge may contain demonstrably 



