84. WHAT 18 LIFE 



atom is not assured sole occupancy of its domain. 

 According to Millikan, "the notion that an atom 

 can appropriate to itself all the space within its 

 boundaries to the exclusion of all other atoms is 

 then altogether exploded . . . . "^ 



The atom is an exceedingly open and loose struc- 

 ture. It is generally agreed that if the constituents 

 of the atom were packed close together, they would 

 occupy only an infinitesimal part of the volume that 

 is the volume of the atom. As Millikan has shown, 

 a wall of lead at least sixteen feet thick would be re- 

 quired to absorb the "cosmic" rays. In his new 

 cathode tube, Coolidge passes a stream of countless 

 billions of electrons through a window that is made of 

 a nickel plate about 500,000 layers of nickel atoms 

 thick (although only one-half of one-thousandth inch 

 in thickness), and only at rare intervals does an elec- 

 tron collide with an atom in the passage though the 

 500,000 layers. 



The alpha particle that is emitted by radium, 

 and that is 8,000 times more massive than an 

 electron, shoots through about 130,000 molecules of 

 air before being stopped. All the evidence forces to 

 the conclusion that, as Millikan says, "the atom 

 itself must consist mostly of 'hole'; in other words, 

 that an atom, like our solar system, must be an ex- 



' The Electron, second edition, 194. 



