We have seen that the study of the spectrum led Maxwell to conclude not only that the atoms were identical in weight and form but that they were the only permanent and indestructible units in this changing world. The apparent identity of the spectrum under all conditions certainly strongly supported such a view at that time. It was believed that if some of the atoms were changing, it would be shown by a gradual alteration of their modes of vibration, i.e., of the spectrum. It was left to the beginning of this century to show the fallacy in this deduction, and to bring undoubted evidence that some elements at least are undergoing spontaneous transformation with the appearance of new types of matter giving a new and characteristic spectrum. This question will be discussed later in some detail.
Electrons
Before, however, considering the bearing of radioactive phenomena on the structure of the atom, I must refer to a discovery which has exercised a most profound influence on the development of physics in general and on our ideas of the structure of atoms. Sir William Crookes long ago found that when an electric discharge was passed through a vacuum tube at very low pressures, a peculiar type of radiation appeared, known as the "cathode rays." This radiation appeared to be projected from the cathode in straight lines, and, unlike light, was deflected by a magnet. These rays excited strong phosphorescence in many substances in which they fell, and also produced marked heating effects. Crookes concluded that the cathode rays consisted of a stream of negatively charged particles moving at high speed. The general properties of this radiation appeared so remarkable that Crookes concluded that the material constituting the cathode stream corresponded to a "new or fourth state of matter." After a controversy extending over twenty years, the true nature of these rays was finally independently shown in 1897 by the experiments of Weichert and Sir J. J. Thomson. They proved, as Crookes had surmised, that the rays consisted of a stream of negatively charged particles travelling with enormous velocities from 10,000 to 100,000 miles a second, depending on the potential applied to the vacuum tube. In addition, it was found that the mass of the particle was exceedingly small, about 1/1800 of the mass of the hydrogen atom—the lightest atom known to science. These results were soon confirmed and widely extended. These corpuscles, or electrons, as they are now termed, were found to be liberated from matter not only in an electric discharge but by a variety of other agencies; for example, from a metal on which ultra-violet light falls, and also in enormous numbers from an incandescent body. Radium and other radioactive substances were found to emit them spontaneously at much greater speeds than those observed in a vacuum tube. It thus appeared that the electrons must be a constituent of the atoms of mat-
