THE GROWTH OF PHYSICAL IDEAS 111 



The nature of the x-rays was a subject of discussion for 

 many years after tlieir discovery. It seemed equally probable 

 that the x-rays consisted of streams of particles having some 

 analogy to the cathode rays and that they might be waves 

 similar to light waves. The x-rays could not be refracted, as 

 light is, by dense media, and for a time all attempts to diffract 

 them failed. Finally, Max von Laue, the director of the In- 

 stitute of Theoretical Physics in Berlin, showed that a dif- 

 fraction pattern could be produced from a beam of x-rays by 

 the use of a natural crystal. It was generally agreed that 

 x-rays represented an electromagnetic radiation similar to 

 that of light but of much shorter wave length, the x-rays from 

 a tungsten target having a wave length about one five- 

 thousandth that of visible light. 



The discovery of the x-rays was follow^ed by the discovery 

 of radioactivity and the identification of the alpha particles 

 emitted by radium with doubly charged helium atoms by Sir 

 Ernest Rutherford.* This work on radioactivity focused 

 Rutherford's attention on the structure of the atom, and in 

 1913 he suggested that atoms were made up of a nucleus con- 

 taining practically the whole of the mass of the atom and a 

 number of electrons rotating in orbits around the central 

 nucleus which were sufficient to neutralize the charge on the 

 nucleus and thus insure an electrically neutral atom. Ruther- 

 ford was led to this view of the structure of the atom by ex- 

 periments on the deviation of rays, particularly the alpha 

 rays, when they collided with atoms, just as something could 

 be learned about the shape of a building by the way in which 

 balls thrown at it bounced. 



At the same time, Niels Bohr w^as studying another prop- 

 erty of atoms, the spectra ^vhich they emit when they are 

 excited by the passage of electricity. When atoms are excited 

 electrically, as gases in a vacuum tube or an electric arc for 

 instance, they emit spectra w^hich are not continuous, like 

 those of hot bodies, but consist of isolated lines. Some of 



* Chapter VI, p. 136. 



