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Joseph John Thomson, D.Sc, F.R.S., in the Cavendish Laboratory, Cambridge, England
to fill completely the space occupied by a hydrogen or any other atom.
Inside a hydrogen atom electrons are therefore very sparsely distributed, for there is manifestly plenty of room for 800; more room indeed than there is in the solar system for the sun and planets; but some atoms contain many more than this number, and the tightest packing known exists in the atoms of the radioactive substances, Uranium, Radium, and the like, each atom of which contains something like two hundred thousand electrons. Even this is very far from tight packing, the intervening spaces are still very great compared with their size, but they are getting too crowded to be comfortable, and nature does not seem to have evolved any permanent atom more tightly packed than these. Moreover even these are not quite stable and permanent, every now and then a particle escapes and flies away, from one or another atom, into space; so that if we take a perceptible quantity of the substance—which of course consists of many billion atoms—a considerable number of particles are always being shot off from it; hence a substance composed of these heavy atoms maintains a continuous bombardment, emitting rays analogous to those which Crookes had so strikingly exhibited in 1879 in an exceptionally high vacuum tube. The experimental discovery of spontaneous radioactivity is due to M. Henri Becquerel in Paris in the year 1896, one year after Roentgen's singular discovery of the existence and electrical generation of X-rays.
Our present view of an atom of matter therefore is something like the following:—Picture to one's self an individualized mass of positive electricity, diffused uniformly over a space as big as an atom,—say a sphere of which two hundred million could lie edge to edge in an inch, or such that a million million million million could be crowded tightly together into an apothecary's grain. Then imagine, disseminated throughout this small spherical region, a number of minute specks of negative electricity, all exactly alike, and all flying about vigorously, each of them repelling every other, but all attracted and kept in their orbits by the mass of positive electricity in
