2 Professor William H. Bragg [Jan. 27^ 



gases consist : in the case of radioactivity, the things which move are 

 quite different. Tiiey are sometimes electrons, vs^hich have come to 

 be called y8 rays when their speed is great, and cathode rays when 

 it is somewhat less ; or they are 7, or x rays, which are new things to 

 us ; or if as a particles they are helium atoms, such as we have known 

 before, they move with excessive speeds which give them quite new 

 properties. In general the radiant particles move hundreds of 

 thousands of times as fast as the gas molecules do, and it is, no 

 doubt, on account of this fact, as well as through their usually 

 extreme minuteness, that their power of penetrating matter is so great. 

 When two molecules of a gas collide, they approach within a fairly 

 definite distance, which we call the sum of the radii of the molecules : 

 and the approach is followed by a recession and new conditions of 

 motion. Each molecule lias, as it were, a domain into which no other 

 molecule can penetrate. But the defences which guard the domain 

 are of no account to the vigorous movements which we are considering 

 now. The radiant particles pass freely through the atoms, and their 

 encounters are rather with one or other of a number of circumscribed 

 and powerful centres of force which exist within the atomic domain, 

 and act with great power when and only when approached within 

 distances which are small in comparison with the atomic radius. It 

 is on this account that the new theory opens out to us such possi- 

 bilities of discovering the arrangement of the interior of the atom. 

 Never before have we been able to pass anything through an atom : 

 our spies have always been turned back from the frontier. Now we 

 can at pleasure cause to pass through any atom an a particle, which 

 is an atom of helium, or a ^ particle, which is an electron, or a y, or 

 X ray, and see what has happened to the particle when it emerges 

 again : and from the treatment which it seems to have received we 

 must try to find out what it met with inside. 



The newer movement exists superimposed upon the other. Its 

 velocities are so great that the gas (or liquid or solid) molecules are, 

 in comparison, perfectly still. There is, as it were, a kinetic theory 

 within a kinetic theory : there is a grosser movement of gas mole- 

 cules which has long been studied, and in the same place and at the 

 same time there is a far subtler and far more lively movement which 

 is practically independent of the other. Your Vice-President, Sir 

 William Crookes, was the first to find any trace of it. The behaviour 

 of the cathode rays in the vacuum tubes which he had made showed 

 him that he was dealing with things in no ordinary condition. What- 

 ever was in motion was neither gas, nor solid, nor liquid, as ordinarily 

 known, and he supposed it must be possible for matter to exist in a 

 fourth state. We have gone far since Sir William's first experiments. 

 The X ray tube and radium have widely increased our knowledge of 

 phenomena parallel to those of the Crookes tube. But I think we 

 may still be glad to use Sir William's definition. 



There is another very striking characteristic of the newer kinetic 



