SINCE the development of the atomic theory on an experimental foundation by Dalton, the progress of chemistry has been based on the central idea of the permanency and indivisibility of the atoms of the elements. The whole experience of chemistry for nearly a century had shown clearly that it was impossible to break up the atoms of the elements by the application of ordinary chemical and physical processes. This idea has had to be modified to some extent by the rapid growth of our knowledge

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... g the last twenty years of the inner constitution of the atoms. It is now generally accepted that the atoms of the different elements have all the same general type of structure. At the centre of the atom is a positively charged nucleus of minute dimensions which is responsible for most of the mass of the atom. This is surrounded by a distribution of electrons held in equilibrium by the forces from the nucleus. The electrons occupy rather than fill a region the diameter of which is of the order of 2 x 10-8 cm. The nuclear charge of the atoms follows a very simple rule first clearly brought to light by Moseley. The resultant nuclear charge of an atom is equal to its atomic or ordinal number and varies from 1 " atom " of electricity in the case of hydrogen to 9 2 atoms in the case of uranium. These ordinal numbers represent also the number of " planetary " electrons, as they have been called, which surround the nucleus of the atom. On this view of the atom, its ordinary physical and chemical properties, apart from its mass, are governed entirely by nuclear charge, for this controls the number and arrangement of the external electrons on which these combining properties mainly depend. -The mass of the atom is a property of the nucleus and exercises only a second-order effect on the distribution of the electrons and so on the ordinary properties of the atom. This point of view offers a t once a simple explanation of isotopes, which consist of atoms of the same nuclear charge but of different nuclear masses. By the action of light and electrical discharges, we can readily remove one or more of the external planetary electrons from the atom, while by the action of X-rays and swift @-rays we may even eject one of the more strongly bound electrons of the system. In this way, we can effect, in a sense,