RADIOACTIVITY 29 



then we find a beautiful illustration of a prominent feature 

 of the radioactive theory. The atom is to be considered as 

 quite an empty thing. It has parts no doubt; we find it best 

 to represent them simply as centres of electric force ; but they 

 are perhaps few and certainly far between, so that one atom 

 sweeps easily through another, with perhaps never a clash. 

 But why is it only now that we find this effect, why have we 

 never discovered it before ? 



Again let us go back to Rutherford's experiment. By 

 passing the a particles through electric and through magnetic 

 fields he found they were deflected in both cases ; and making 

 the usual calculations he found that the velocity was enormous ; 

 e.g. the initial velocity of the a particle from RaC was some 

 20,000 miles a second. At the same time it appeared that the 

 charge the helium atom must be carrying was twice the charge 

 carried by the hydrogen atom in electrolysis. 



Here is a novel enough condition. At the hottest tempera- 

 tures we meet with, the atoms of a gas never move with a 

 speed approaching to this in the slightest degree ; at ordinary 

 temperatures the molecule of hydrogen has a speed of only 

 186,000 cm. a second. This is surely enough to explain matters ; 

 and we ascribe the motion of the molecules through one 

 another to their terrific speed. Physics and chemistry concern 

 themselves with the interplay of atoms which meet surface to 

 surface only : each atom has its own domain which it keeps 

 to itself and has relations with others at its boundaries only. 

 But now we have an entirely different state of things, to which 

 surface actions between atom and atom are of no significance 

 at all. The atoms can pierce each other if they approach with 

 sufficient relative speed and we are very ready to believe that 

 under conditions so new to us we must find effects that are 

 unfamiliar. 



Then does nothing happen to two atoms which pass through 

 each other in this way? There is at least one consequence — 

 viz. that which has been termed ionisation. Some of the 

 atoms through which the a particles pass are found to be 

 charged, though they were neutral before ; the helium 

 atom itself is always charged. It is to be remembered that 

 the latter has many encounters, including the first encounter 

 or shock of its separation from the parent atom. The helium 

 atom is positively charged and we may infer that the atoms 



