lO DISCOVERY AND NATURE OF RADIOACTIVITY 



Beta rays and alpha rays are both vehicles of energ}-. Owing to 

 their high speed and relatively minute size, the particles of the /9 rays 

 may penetrate great numbers of atoms, passing through the spaces 

 between the electrons that compose them. Under such conditions 

 they continue in their path without deflection ; but when a /9 particle 

 collides with an electron the latter may be torn from the atom and set 

 free (ionization). The ion may not always be torn from the atom by 

 the collision, for the impact may serve only to deflect it from its 

 path. In either event the [^ particle will lose energy, will therefore 

 travel more slowly, and hence be thereafter more easily deflected. 



The particles of the a rays are more effective ionizers than the 

 electrons, but, owing to their relatively large size, they are not easily 

 deflected by collision. Hence the a particle loses energy chiefly by 

 collision. 



It is partly for the reasons just described that both the a and the 

 [i rays, as stated above, are complex. They are both composed of 

 streams of particles possessing widely varying amounts of energy. 



The Complexity of the Atom : A conception of the atom 

 such as the discoveries in radioactivity compel us to adopt, has been 

 expressed by Perrin in a very striking figure. He likens the atom 

 to a miniature planetary system. If a suitable force acts on an atom 

 strongly enough it disengages a negative planet, or electron, produc- 

 ing thereby ionization. If the atom is very unwieldly, that is, rela- 

 tively very large, and the corpuscle far from the center — the 

 Neptune of the system — it will be loosely held by the electrical 

 attraction of the remainder of the atom, and so more easily separated 

 from it. In a somewhat speculative calculation, based upon the 

 energy liberated in radioactive processes, Campbell ^^ has estimated 

 that " the number of electrons in a radium atom must be greater, 

 and probably very much greater than 1,200." 



Theory of Atomic Disintegration : The researches of Ruth- 

 erford and Soddy,^-^ of Thomson, ^^^ and of Rutherford all indi- 

 cate that radioactivity is a manifestation of sub-atomic change. "In 

 its simplest form," says Rutherford, ^^^ "the theory (of atomic dis- 

 integration) supposes that every second a certain fraction (usually 

 very small) of the atoms present become unstable and explode * with 



* Sir Oliver Lodge, in a recent discussion of Lord Kelvin's philosophy (Nature 

 76: 198. 2 Jl 1908), has called attention to the difference between the static and the 

 kinetic conceptions of the atom. " The internal energy of Lord Kelvin's model atom 



