74 Scientific Proceedings, Roi/al Dublin Society. 



in certain phases of radioactive cliange. As they are not radiant matter in 

 the elemental sense, they do not now concern us. 



There is, however, another sort of ray coming from the radioactive 

 substance which is stopped by the glass slip, and hence has not, as yet, 

 affected the electroscope. When I now remove the glass slip, there is a rapid 

 — a very rapid — collapse of the leaf. You perceive at once that this sort of 

 ray, although not so penetrating as the /3 or 7 rays, inasmuch as the glass 

 completely stops it, is far more active in discharging the leaf. These highly 

 active rays have been named by Eutherford (to whom we owe the greater 

 part of what we know about the subject) the a rays. 



When I reinsert tlie glass slip, the rapid motion of the leaf ceases 

 instantly, and is replaced by the slow movement due to the penetrating rays. 

 This observation is important. It is known that radium is transformed into 

 a highly radioactive gas — the emanation. Tlie striking effect upon removing 

 the glass might be supposed to be wholly, or in part, due to tlie diffusion or 

 convection of this gas into the electroscope. The fact that the rapid collapse 

 ceases immediately we reinsert the glass slip, negatives this explanation : 

 for if the emanation had got iuto the electroscope, its effect must continue 

 after we closed the opening, seeing tliat this gas preserves its vigorous effects 

 for some days. 



The next experiment 1 shall make is to substitute for the glass slip a very 

 thin cleavage flake of mica. This is not sufficient to completely stop the 

 a rays. We notice a fairly rapid collapse of the leaf. It is evident, then, 

 that while a glass plate one millimetre thick completely stops these rays 

 they pass through the thin flake of mica. Lastly, it is easy to show that a 

 few centimetres of air are quite as effective as the glass plate in stopping these 

 rays. Thus, when I leave the aperture open, but lower the radioactive salt 

 about 8 or 10 centimetres below it, the only perceivable effect is that due to 

 /3 and y rays. 



The explanation "action at a distance," when one body affects another in its 

 neighbourhood, is, fortunately for the progress of science, not now regarded 

 as sufficient. In such cases three alternatives present tliemselves. There may 

 be some change transmitted in the intervening medium (ethereal or material) 

 or there may be something of the nature of projectiles cast out by the 

 active body. If the latter is the explanation, we have to admit that 

 these projectiles can pass through solid matter as we have seen tlie ^ rays 

 readily do. 



Not having time to enter upon the sequence of observations which led to 

 the elucidation of these rays, I must briefly state that the projectile theory is 

 now universally accepted, and tliat the truly atomic nature of the a ray has 



