4 Professor William H. Bragg [Jan. 27, 



encounter takes place. When it is over there are still two things, 

 an atom and a radiant particle going away from it. The sum of 

 the energies of the two is still the same, which means that we deny 

 a possibiHty much considered at one time — viz. that in the encounter 

 the atom could be made radioactive, and could unlock a store of 

 energy usually unavailable. We suppose there is no energy to be 

 considered except the original energy of the radiant particle, and we 

 suppose that there are not now two or more radiant particles in place 

 of the original one, which also is a limitation on previous ideas. It 

 is a theory which ascribes a corpuscular form to all the radiations. 

 Each particle, a, /S, 7 or x, is to be followed from its origin to its 

 disappearance, and we have nothing to think of but the one particle 

 threading its way through the atoms. It loses energy as it goes, 

 though little at any one collision, and it passes out of our reckoning 

 when it has lost it all. There are no secondary radiations other than 

 radiant particles moving in directions which are different from those 

 in which they moved at first. Even when a cathode ray excites an 

 X ray in the ordinary Rontgen tube, or the x ray excites a cathode 

 ray in a manner almost as well known, it is hardly an exception to 

 this rule. The cathode ray has an encounter with an atom and dis- 

 appears ; simultaneously the x ray comes out of the atom, a circum- 

 scribed corpuscle carrying on the energy of the cathode ray. There 

 is a change, but it extends only to the external characteristics of the 

 carrier of energy. The x ray passes through the glass w^all of the 

 X ray bulb, or at least it does so sometimes ; it may pass through 

 other matter as well, but sooner or later it has a fatal encounter 

 with an atom, and the reverse change takes place. In all cases, in 

 that of the undeviating a ray, or the yS ray which suffers so many 

 deflections, or the y or 2; rays, it is a matter of tracing tlie movements 

 of individual minute quantities of energy until they finally melt away. 



Let us consider one or two simple experimental results from this 

 point of view in order that we may illustrate this corpuscular theory, 

 and at the same time may learn something of the properties of the 

 corpuscles and of the arrangements of the atoms through which 

 they pass. 



We take first one of the simpler cases, the movement of an a 

 particle through a gas. The relatively large mass of the particle gives 

 it an effectiveness which the other radiations do not possess. It 

 moves straight through every atom it meets, and ionises most of 

 them. Very rarely does it suffer any deflection from its course until 

 its velocity is nearly run down. Then indeed it does appear to depart 

 considerably from the straight path, and it may be that it is much 

 knocked about by collisions liefore it finally comes to comparative 

 rest. In this way we may explain the distribution of the ionisation 

 along its path, which increases slowly at first and rapidly afterwards, 

 until the a particle has nearly finished its journey : it then falls off 

 rapidly. Considering that the ionisation increases as the particle 



