1917] on The Complexity of the Chemical Elements 135 



the necessary preliminary to the nnderstandiiig of the third aspect 

 in which the elements are or may be complex. That uranium and 

 thorium are built up of different isotopes of lead, helium and electrons 

 is now an experimental fact, since they have been proved to change 

 into these constituents. But the questions how they are built up, 

 and what is the nature of the non-radioactive elements, which do not 

 undergo changes, remain unsolved. 



Professor Bragg showed in 1905 that the ^-particles can traverse 

 the atoms of matter in their path almost as though they were not 

 there. As far as he could tell, and the statement is still true of the 

 vast majority of a-particles colhding with the atoms of matter, the 

 a-particle ploughs its way straight through, pursuing a practically 

 rectilinear course, losing slightly in kinetic energy at each encounter 

 with an atom, until its velocity is reduced to the point at which it 

 can no longer be detected. From that time, the rt-particle became 

 as it were, a messenger that could penetrate the atom, traverse 

 regions which hitherto had been bolted and barred from human 

 curiosity, and on re-emerging could be questioned, as it was questioned, 

 effectively by Rutherford, with regard to what was inside. Sir J. J. 

 Thomson, using the electron as the messenger, had obtained valuable 

 information as to the number of electrons in the atom, but the 

 massive material a-particle alone can disclose the material atom. It 

 was found that, though the vast majority of a-particles re-emerge, 

 from their encounters with the atoms, practically in the same direc- 

 tion as they started, suffering only slight hither and thither scattering 

 due to their collisions with the electrons in the atom, a minute 

 proportion of them suffer very large and abrupt changes of direction. 

 Some are swung round, emerging in the opposite to their original 

 direction. The vast majority, that get through all but undeflected, 

 have met nothing in their passage save electrons, 8000 times lighter 

 than themselves. The few, that are violently swung out of their 

 course, must have been in collision with an exceedingly massive 

 nucleus in the atom, occupying only an insignificant fraction of its 

 total volume. The atomic volume is the total volume swept out by 

 systems of electrons in orbits of revolution round the nucleus, and 

 beyond these rings or shells guarding the nucleus it is ordinarily 

 impossiljle to penetrate. The nucleus is regarded by Rutherford as 

 carrying a single concentrated positive charge, equal and opposite to 

 that of the sum of the electrons. 



Chemical phenomena deal almost certainly with the outermost 

 system of detachable or valency electrons alone, the loss or gain of 

 which conditions chemical combining power. Light spectra originate 

 probably in the same region, though possibly more systems of 

 electrons than the outermost may contribute, while the X-rays and 

 y-rays seem to take their rise in a deep-seated ring or shell around 

 the nucleus. But mass phenomena, all but an insignificant fraction, 

 originate in the nucleus. 



