1911] on a Fourth State of flatter 3 



theory which differentiates it sharply from the older. The experi- 

 ences' of any one of the radiant particles in an atom which it crosses 

 are quite unaffected by any chemical combination of that atom with 

 others ; that is to say, by any molecular associations it may have. 

 Naturally, this simplifies investigation. We may no doubt ascril)e 

 this state of things to the fact that a radiant particle is concerned 

 rather with the interior of the atom than with the exterior, and that 

 it is the latter which is of importance in chemical action. 



Let us take notice of one more important difference. The mole- 

 cules of a gas move with velocities which vary at every collision, yet 

 vary about a certain mean. But the peculiar motion of the radiant 

 particle is only temporary. For only a very short time can any ray 

 be described as matter in a fourth state ; at the end of it the extra- 

 ordinary condition has terminated, the particle has lost its tremendous 

 speed or suffered some other change, and the ray ceases to exist. 

 Speaking technically, we are dealing with initial not permanent 

 conditions. 



Let us now come back to resemblances between the two kinds of 

 motion ; for there is one point of similarity which is not quite so 

 obvious as others I have mentioned, and is, I think, of the greatest 

 importance ; in fact, it is largely on account of this similarity that I 

 have ventured to put the two theories together for comparison. 



When the first experimenters in radioactivity allowed their streams 

 of rays to fall upon materials of various kinds, they found tliat the 

 irradiated surfaces w^re the sources of fresh streams of radiation. 

 The secondary rays were sometiuies of the same nature and quality 

 as the primary, sometimes not. Further, they found that the 

 secondaries, on striking material substances, could produce tertiaries, 

 and so on. The examination of all the variations of this problem — 

 the investigation of the consequences of changing the primary, of 

 changing the substance, and last, but not least, of changing the form 

 of the experimental arrangements — has been the cause of an enor- 

 mous amount of work. There is a large literature dealing with 

 secondary radiations of all kinds, which, I imagine, but few have read 

 with any completeness, and the subject has become, on the surface at 

 least, complicated and difficult. Now I believe that it is possible to 

 clear away the greater portion of this complexity at a stroke, by the 

 adoption of an idea which makes it possible to describe and discuss the 

 whole of these phenomena in a very simple way. When an encounter 

 takes place between two gas molecules, we suppose that the sum of 

 the energies of the two is the same after the collision as before, and, 

 further, that there are just two things to consider — two molecules — 

 after as well as before. I think that we may carry this idea over 

 almost bodily to the newer theory. A radiant particle encounters an 

 atom. The particle is a definite thing, it contains a definite amount 

 of energy, and whether it is an a or ^ or 7 or x ray, its energy is to 

 be found almost entirely inside a very minute volume. The 



B 2 



