MATTER, SPACE, AND TIME 239 



the electrons, and affect their radiation, which 

 depends on the rate of acceleration of their motion. 

 The theory of this effect was considered by Lorentz 

 and Larmor, who predicted the subdivision of the 

 spectral lines, afterwards experimentally discovered 

 by Zeeman. 



The connection of the electron theory with 

 the phenomena of radio-activity has already been 

 considered in the aspects which were first appreci- 

 ated. The conception of an atom as a system of 

 electrons in rapid orbital motion naturally suggests 

 its occasional disintegration ; indeed the possibility 

 of such disintegration had been treated as a 

 difficulty of the theory by Larmor before the 

 discovery of radio-activity directly indicated its 

 occurrence. But we now know much more about 

 the modus operandi of atomic disintegration, and 

 have discovered that the changes concerned in 

 radio-activity have not to do with planetary 

 electrons, but with a much more deep-seated 

 and essential part of the atom, its nucleus. 



The first step in the formulation of the modern 

 theory of the atom was the discovery of the 

 electron, the negative electric unit. The second 

 step was the recognition of a positive nucleus. 

 This step was taken by Rutherford. 



The a particles, as we have seen on page 1 93, are 

 helium atoms projected with high velocities. Their 

 mass is four times that of a hydrogen atom, that 

 is, 4 X 1850 or 7400 times the mass of an electron. 

 The forces exerted on a particles by electrons would 

 be much too small to affect their paths appreci- 

 ably. And most a rays, it is true, pursue a straight 

 path through matter. Yet here and there one is 



