SECTION A.— MATHEMATICAL AND PHYSICAL SCIENCES. 



LOGIC AND PROBABILITY IN 

 PHYSICS 



ADDRESS BY 



Dr. C. G. DARWIN, F.R.S., 



PRESIDENT OF THE SECTION. 



Before coming to my main subject it is appropriate that I should begin 

 by referring to the quite exceptional loss that physical science has in- 

 curred during the past year in the death of Lord Rutherford. It is not 

 easy for contemporaries to judge what the future estimates of history 

 will be, but in this case I think we may expect the verdict that he was 

 the greatest of all experimental physicists — -perhaps with the exception 

 of Faraday. It was my good fortune to serve in his laboratory in Man- 

 chester during those days when he had got the subject of radioactivity 

 into fair order, and was profiting by it to explore the structure of matter, 

 the days when the a-particle was giving its best as a probe into the nature 

 of atoms. I suppose that the most important thing that came out was the 

 discovery of the nucleus. This arose out of investigations into the 

 scattering of a-particles in a sheet of gold foil. A few of them were 

 scattered through very much larger angles than they had any right to 

 be, and from this hint Rutherford guessed that the atom must contain 

 a powerful centre of force. I remember the occasion of a Sunday evening 

 supper when he told us about it, in fact only a few minutes after he had 

 worked out the consequences, and I remember being astonished at the 

 use he could make of the vague recollections of what he had learnt at 

 school about the hyperbola. It is easy for us now to say how reasonable 

 it all was, because we have got used to atoms like that, but we have to 

 remember that at the time it certainly caused a great deal of trouble. 

 One single very rare phenomenon was explained, but every critical mind 

 could point out endless objections. For example, there was the prime 

 difficulty that until the advent of Bohr's theory there was nothing to 

 hold the nuclear atom to a fixed size, a difficulty which was explained 

 at any rate roughly by the older idea of a sphere of positive electricity ; 

 and then there was the trouble, which in fact worried Rutherford a good 

 deal, as to how a P-particle could ever escape from such a strong attrac- 

 tive force. But he got it right ; it was a process I have heard described 

 by saying that if Rutherford went into a chemical laboratory for a reagent 

 he would somehow always go to the right bottle even if there were no 

 labels. Of his later work, disintegration and so on, I will not speak, 

 but only refer to one characteristic he showed in it. This was his capacity 



