INAUGURAL. ADDRESS. 



11 



going- such a distance, for a general direction is maintained through- 

 out, as we have seen. We have but to picture in our minds the radia- 

 tion of the boughs and twigs of a tree to realise that a very few 

 successive deviations at random are sufficient to destroy all connection 

 with an original direction. But if the alpha and beta particles re- 

 coiled or Avere deflected in striking the exterior of the atoms which 

 they met, they Avould experience perhaps a hundred thousand such 

 deviations in the first centimetre. The particles, therefore, cannot 

 have encounters with atoms as a whole; they must penetrate them, 

 and usually without any deflection. Only occasionally they must come 

 into encounter with parts of atoms, but probably this does not happen 

 in the case of a beta paiticle once in a hundred or thousand atoms 

 traversed. The actual figure depends upon its speed, because the so- 

 called encounter of the flying particle witli the part of the atom really 

 consists in an approach so close as to permit a sufficient mutual 

 action ; and the faster the particle the nearer the approach has to 

 be in order to produce a given deflection. The problem is exactly that 

 of a comet flying round a sun ; the course of the comet is the more 

 iiltered the nearer the two bodies approach each other. 



All this amoimts to saying that the atoms must be very empty 

 things ; something like solar systems in miniature, a few significant 

 points or parts, and in between a relatively large amount of almost 

 unmeaning space. We are almost out of sight of the original view of 

 the atom, a circumscribed body, into whose interior nothing else could 

 penetrate, occupying so much space to the exclusion of everything 

 else. Such a view was all that was needed in order to explain to us 

 the ordinary physical and chemical eft'ects, such as, for example, the 

 collision of the molecules of a gas. But now the interior of the atom 

 is no longer a forbidden country, the new radiations pass through the 

 atoms with ease. We may look on such transits as journeys of ex- 

 ploration, and hope to leam something of the nature of the interior of 

 the atom from the way in which the motion of the particles has been 

 altered in going through. In the older physics all the actions which 

 we studied, depended on the external presentments of the atoms to 

 each other, and we, therefore, learnt only of their external charac- 

 teristics. Now for the first time we can, as it were, insert something 

 material into the interior of the atom and prove its contents. 



Tt may be useful to put the matter in a slightly different way. 

 The molecules of a gas move rapidly to and fro, changing- their 

 directions and speeds at each mutual encounter. Each molecule of the 

 air in this room moves on the average about one hundred thousandth 

 d[" a centimetre between successive encounters. This distance we call 



