TEE CONSTITUTION OF MATTER 113 



We have seen that' the study of the spectrum led Maxwell to con- 

 clude 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 con- 

 ditions 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 radia- 

 tion 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 heat- 

 ing effects. Crookes concluded that the cathode rays consisted of a 

 stream of negatively charged particles moving at high speed. The gen- 

 eral 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 independent- 

 ly shown in 1897 by the experiments of Weichert and Sir J. J. Thom- 

 son. 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 re- 

 sults 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 agen- 

 cies; for example, from a metal on which ultra-violet light falls, and 

 also in enormous numbers from an incandescent body. Eadium 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- 



