488 POPULAR SCIENCE MONTHLY. 



mental evidence. However, the experiments can not be said to have 

 gone so far as to render its correctness even probable. This much, 

 however, it is safe to say: the experiments upon cathode rays have 

 proved conclusively that under some circumstances particles do exist 

 which are smaller than the ordinary atoms of chemistry. It was the 

 study of cathode rays, then, which first sounded the death-knell of the 

 indivisible atom of our earlier chemistry and prepared the way for the 

 discoveries, which were soon to follow, of subatomic transmutations 

 which involve the liberation of stored-up energies, the very existence 

 of which had never before been dreamed of . 



The Nature of X-rays. 

 I have already said that cathode rays are very intimately connected 

 with X-rays, for both are associated with the discharge of electricity 

 in exhausted tubes. In fact, at the time of Eontgen's discovery, many 

 physicists thought that the X-rays were nothing more nor less than 

 cathode rays which had passed through the walls of the tube into the 

 outside air. But Professor Eontgen demonstrated that the X-rays are 

 wholly different from the cathode rays in these two important respects, 

 namely: (1) they are not deflected in the slightest degree, either by a 

 magnet or by bodies charged with static electricity; (2) they do not 

 impart negative charges to objects upon which they fall. X-rays are 

 therefore not cathode rays. They originate at the point at which the 

 cathode rays strike against the walls of the tube, or against any object 

 placed in their path inside the tube. In the ordinary X-ray tube a 

 little plate of platinum is commonly placed in the middle of the tube, 

 just opposite the cathode, for the purpose of receiving the stream of 

 cathode rays. It thus becomes the source from which the X-rays pro- 

 ceed. This is about all that we know with certainty concerning X-rays. 

 Most physicists, however, now believe them to be ethereal rather than 

 material in their nature, that is, they believe them to be some sort of 

 waves or pulses in the ether, not very dissimilar from light waves. 



Radio-active Substances emit Cathode Rays. 

 We are now in a position to understand the experiments which were 

 performed with radio-active substances, namely, uranium, thorium 

 and radium, in order to discover the nature of their radiation. It was 

 at first suspected that these rays were similar to X-rays, because, like 

 them, they possessed the power of penetrating opaque objects and of 

 affecting photographic plates. But as soon as the test which distin- 

 guished X-rays from cathode rays was applied, that is, as soon as a 

 magnet was placed so that it could distort the photograph produced 

 with the aid of Becquerel rays, in case these rays like cathode rays were 

 deflected by it, it was found indeed that these photographs did indicate 

 such deflection. It was further found that they could be bent out of 



