12 INAUGURAL ADDRESS. 



the mean free path. Now, in the same way, it becomes clear that these 

 particles, the motion of which constitutes the new radiation, move 

 rapidly to and fro, only their velocities are enormously greater than 

 the velocities of the gas molecules. The molecules collide with each 

 other with very little intei^enetration, if any ; these particles take no 

 account of molecular boundaries, but penetrate within, and their 

 colli-sions are with parts of atoms, not with the atoms as wholes. The 

 mean free path of a molecule of an ordinary gas is a minute fraction 

 of a centimetre. The paths of these particles between successive 

 encounters at which the direction of their motions are violently 

 changed, is immensely gi^eater, varying from a millimetre or so in 

 the case of cathode rays to a metre in the case of the beta rays, whilst 

 the gamma rays can penetrate hundreds of yards of ordinary air 

 without being turned aside. 



To some extent, we had already learnt to recognise the penetra- 

 bility of tlie atom before the discovery of radio-activity. Hertz and 

 Lenai-d had shown us that cathode rays could penetrate extremely thin 

 metal sheets. But the new facts are a revelation to us in this direc- 

 tion. The beta rays, which move five to ten timas as fast as the 

 cathode rays, have a thousand times the penetrating power. And 

 more singular still is the penetration of atoms by the alpha particle, 

 itself an atom. That two atoms can for a moment occupy the same 

 space is certainly a novel and instructive conception. Of course, the 

 singular penetrating power is due to enormous speed. In helium, at 

 ordinary pressures and temperatures, the atoms do not penetrate into 

 each other at all; it is only the helium atom which is ejected from 

 radio-active atoms at terrific speeds which does not respect the atomic 

 boundaries. 



Equally striking is the penetrating power of the Rontgen or 

 gamma ray, whatever view may be taken as to its nature. The older 

 hypothesis gives it the nature of a pulse or irreg-ular disturbance of 

 the aether. It has been modified in order to fit recent experimentvS ; J. 

 J. Thomson now conceives of it as a tiny " bundle of energy," possess- 

 ing almost all the properties of a material particle. I have, myself, 

 ventured to take the simpler view that it really is a material particle, 

 as I will explain a little later. For the present I will anticipate only 

 so far as to assume what is consistent with either liypothesis — ■^'iz., 

 that the ray consists of something in motion in a straight line, 

 possessing energy, and having boundaries wliich must be far more 

 restricted than those of an atom, and must, probably, be comparable 

 with those of an electron. Now imagine such an entity passing in a 



