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 Röntgen'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 Röntgen 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 proceed. 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 distinguished 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
