The final selections of this volume will examine the downfall of notion of atoms as indivisible. The atom as an ultimate and therefore indivisible particle of matter was a venerable and a viable scientific notion for many years [Newton 1704]. That is not to say that the indestructibility of the atom was universally accepted by scientists in the 19th century. Indeed, not even the existence of the atom was universally accepted for most of the century! Furthermore, some physicists were already trying to formulate models of atoms before any of its constituent particles had been identified.
The selections of this section will look at some of the evidence for the divisibility and impermanence of atoms accumulated in the late 19th and early 20th centuries. The celebrated characterization of cathode rays by J. J. Thomson in 1897 identified a universal constituent of atoms, the electron. The discovery of radioactivity by Henri Becquerel in 1896 inaugurated the study of the atomic nucleus, even though strong evidence for the nuclear atom would have to wait another 15 years. This section will look at Becquerel's new phenomenon and its systematic early study by Marie Curie. Several years and many studies later, it was apparent that radioactivity involved disintegrating atoms. Ernest Rutherford's characterization of one of the common radioactive fragments, the alpha (α) particle, is an instructive tale. The section concludes with a review article by Frederick Soddy which elucidates the various transformations of natural alpha and beta radioactivity, and discusses isotopy to boot.
Of course the story of atomic structure does not stop there. The discovery of the electron was only the first step in unravelling the role electrons play in the structure of atoms and, much more interesting to chemists, the binding of atoms together into molecules. Similarly, natural radioactivity was the phenomenon which proved to be the gateway to nuclear physics, an area which would profoundly affect the 20th-century's science and history.
Unfortunately, such developments lie outside the scope of this book, or at least outside the skill of this author. The theoretical development of quantum mechanics and quantum field theories embrace an ever-increasing mathematical sophistication. At the same time, the experimental detection of new particles or the imaging of single atoms are accomplished throught the instrumentality of ever more complex machinery. I do not wish to imply that these chapters of science are beyond the reach of the interested layperson, and I applaud the many imaginative and successful authors who have interpreted the efforts of these specialists to non-specialists. Still, I maintain most of quantum chemistry and nuclear physics is accessible to non-specialists primarily through re-telling or re-working communications by specialists rather than through the close reading of the original works which I have attempted in this collection.