most important and remarkable discoveries will be awaited with the greatest interest.

With regard to the four elements—lead, thorium, bismuth, and barium-it has been quite recently shown by Boltwood that the latter three elements do not satisfy the quantitative conditions consequent on their being produced from uranium. With lead the case would, however, appear to be different, for the quantity of this element appears from a large number of analyses to bear a constant ratio to the quantity of uranium with which it is associated in primary minerals of the same age. On the other hand, those minerals which are known to be of greater antiquity contain a greater relative portion of lead than those known to have been more recently formed, These circumstances are interpreted by Boltwood as indicating that lead is the final disintegration product of uranium. Strong as this evidence may appear, there are grave difficulties in the way of accepting this conclusion as correct. The atomic weight of lead being 207 and that of uranium 240, there is a change of

only 33 units in the fourteen transformations necessary for the conversion of uranium into lead. In view of the fact that the atomic weight of radium emanation seems to be about 100, the above conclusion appears to be doubtful. A determination of the atomic weight of some of the other transformation products of uranium would therefore be desirable to remove this discrepancy in the evidence on the question.

CHAPTER X

RADIO-ACTIVITY AS A GENERAL PROPERTY OF

MATTER

INTRODUCTION

THE study of the constituents of minerals, such as pitchblende, containing uranium, has shown that their radio-activity is due not only to the uranium which they contain, but to the whole series of substances exhibiting the property of emitting radiations to a greater or less extent. It will also be remembered that it has been shown that helium is produced by radioactive processes, and that other known chemical elements such as lead are probably also formed as a result of these transformations.

Thus it will be seen that radio-activity is not a property possessed by one form of matter only, and it becomes a question of interest not only to consider whether these

non-radioactive products of the radioactive elements are really completely devoid of the properties of the substances from which they are known to have been formed, but also to investigate the question whether the property of radio-activity is not at least in some measure shared by all known chemical elements.

Now radioactive transformations can be detected only by the radiations to which they give rise, so that if we are deprived of the means of detecting these radiations we are left powerless to investigate these subtle changes taking place in the atoms of the elements. That there may be many radioactive processes which leave us in ignorance of their existence is obvious from the following considerations.

It was shown when dealing with the a radiation emitted by radioactive bodies that if the velocity of the positively charged particle projected from the active substance fell below a certain limit, the radiations ceased to produce any ionization in a gas or any action on a photographic plate or

phosphorescent screen placed in their path. When the a particles had traversed a certain definite distance in air, called by Bragg their "range," their velocity was so far reduced that they ceased to produce any electrical, photographic or phosphorescent effect by which their presence could be detected. Hence, though it was certain that the particles were being projected with enormous velocities through the space beyond their range, their presence could not be detected by any known method. It follows, therefore, at once that if a particles were to be projected from a radioactive atom with a velocity even slightly less than that with which the a particles are emitted in the cases we have hitherto been considering, we might be wholly unconscious of their existence.

Whether such radiations do, in fact, arise from substances usually considered to be inactive must at present remain a matter merely of speculation; the question can be answered only by a refinement of our methods of detecting radio-activity or by some entirely new mode of investigating