128 REPORTS ON THE STATE OF SCIENCE. 



means to discover or investigate it. Three separate lines of evidence 

 which have lately transpired show this in the clearest possible manner. 

 In the first place, Rutherford proved the existence of changes in the 

 disintegration series of radium, thorium, and actinium in which no detect- 

 able radiation was expelled. Let A change into B, and B into C, 

 and C into D. The middle change of B into C might be a ' rayless ' 

 change, but it would still be possible to detect it so long as detectable 

 radiation were exjielled from A and C. The same point may be 

 illustrated also by a slightly different case taken from an investigation 

 four yeai's ago in a series where some of the changes only emit « rays, 

 and others both a and y8 rays. If some of the radium emanation which 

 emits a rays, but not /3 rays, is put into a vessel with thin walls, capable 

 of absorbing completely the a radiation and tdl owing the /j radiation to 

 pass through, no external radiation can be detected from the vessel when the 

 emanation is first introduced. Little by little, as the emanation begins to 

 change, an external radiation comes from the vessel and gradually grows. 

 If now the emanation is blown out of the vessel, the products of its 

 change, being solid, are left behind, deposited on the inside walls. The 

 external radiation is not at first affected, but as time goes on gradually 

 decays away. It would not have made any difference to the above 

 experiment if the emanation had not given out rays. Its existence could 

 have been inferred from its producing a body which did give out rays, 

 and, moreover, the rate of its change into this body could be determined. 

 In this way Rutherford has established that changes occur without the 

 emission of a detectable amount of energy, but which are, nevertheless, 

 demonstrable, because they are preceded and followed by changes in 

 which detectable energy is given out. 



In the next place, the past two years' work on the nature of the a lay 

 has shown clearly that it is only detectable within somewhat narrow 

 limits of velocity. If we represent the velocity of the fastest moving 

 o particle expelled from radium by 100, then it has been shown that an 

 a particle moving with a velocity below 43 would fail to be detected by all 

 our methods — electrical, photographic, or phosphorescent. The identical 

 a jmrticle, we know through radioactive tests, might be expelled with a 

 velocity up to 6,000 miles a second, and we should be unaware of it and 

 incapable of detecting it by its energy. 



Lastly, the possibility remains to be considered that such a particle 

 might-be detectable by its charge. I need only mention the ingenious 

 contrivance of Strutt, popularly known as the radium clock, to illustrate 

 a possible method. This detects the loss by radium of negative electricity 

 carried away by the /3 particles which are negatively charged. In the 

 same way it might be thought that an otherwise undetectable a radiation 

 might be detected by the loss of positive electricity. This hope, however, 

 proves illusory. There are theoretical grounds which made me suspect 

 that the positive charge carried by the n particle is an accident of the 

 conditions under which it is investigated, and that the a particle is not 

 charged when initially expelled from the atom. This I have now proved 

 to be the case, and, so far as we can see, if its energy was below that at 

 which it could be detected the « particle would never become charged 

 at all. 



So that unless entirely new methods of investigation are discovered 

 we are forced back in the search for direct evidence of other processes 

 of evolution on the purely material methods, such as the use of the 



