INAUGURAL ADDRESS. 13 



straight line thi'ough all the molecules in, say, a hundred yards of air 

 without suffering deflection, indeed without suffering any loss of energy 

 at all, as can easily be shown to be the case. What can it have met 

 'with on the way? It cannot have had an encounter with anything 

 forbidding it admission into any region ; it can hardly have pushed 

 things out of the way, or it would have lost energy. If there were any 

 centres at all in any of the atoms which were impenetrable, the ray 

 would have met not one but many of them. Extraordinary as it may 

 be, it really seems that penetration to the uttermost is only a matter 

 of degree, that there is nothing, not even of the minutest kind which 

 occupies a definite portion of space to the exclusion of everything else. 

 Of coui"se, a statement like this sufi'ers from the vagueness which is 

 almost unavoidable when new ideas are put into old words. But I am 

 trying to show that we must put aside the older conception of the 

 properties of matter. At one time two bodies could not fill the same 

 space; with the recognition of atomic composition came the under- 

 standing that there was space between the atoms of which bodies were 

 compased; later came the idea of the penetration of the atom, and 

 there were only certain electrons within the atoms which kept, portions 

 of space to themselves. We have always been pushing the limits back, 

 it does not seem unlikely now that there are no limits at all, and that 

 we might conceive of radiation having any desired penetration. At 

 the present time the hard gamma rays are the most penetrating of 

 those that we know. As I have already said, they fly on the average 

 through great thicknesses of matter, hundreds of yards of air, or 

 inches of lead, before suffering serious deflection at some encounter 

 with a part of the internal structure of the atom. Only an encounter 

 is not merely a geometrical relation between spaces occupied by the 

 two encountering particles. 



It is impossible to pass by the conception of penetrating radia- 

 tion without considering two other problems which have led, or might 

 lead, to the assumption of its existence. A long time ago the Genevan 

 philosopher, Lesage, filled all space with a penetrating radiation moving 

 in all directions for the express pui'pose of accounting for gravitation. 

 The attraction of two bodies for one another was to be ascribed to the 

 shelter which each gave to the other from the driving streams. Since 

 the whole of a body counts in the attraction which it exerts, the inside 

 of the body must contribute to the sheltering as well as the outside, 

 and this requires a radiation of extreme penetration. Some fraction 

 of the radiation is to be timied aside by any body through which it is 

 passing, or there would be no sheltering, but it must be a very minute 

 fraction indeed. It is not an impossible theoiy, and leads to result* 



