the ordinary metals these efforts have all resulted in failure. Thus J. J. Thomson1 was unable to detect the presence of radium in such metals as zinc, tin, lead, bismuth, and copper, and Righi, Wood, and Campbell' all failed to obtain any emanation from a variety of metals investigated by them.

These results dispose of the possibility of the existence of thorium, radium, or actinium in the metals tested. The possibility of the presence of uranium may be discarded, for it can easily be shown that if this metal were present in quantity sufficient to account for the radiations emitted, its presence should be demonstrable by chemical means, and this is not the case. In this connection it may be of interest to mention that Elster and Geitels have quite recently detected very minute quantities of polonium in lead; but the amount was so exceedingly minute that it

1 J. J. Thomson, Brit. Assoc. Rep., 1904. 2 Righi, Cim., January, 1905, S. 53.

3 A. Wood, Phil. Mag., April, 1905.

4 N. R. Campbell, Phil. Mag., April, 1905, and February,

1906.

Elster and Geitel, Phys. Zeit., 1907.

required the greatest experimental skill to detect its presence at all, so that this discovery does not affect the general argument regarding the radiations emitted from ordinary metals.

Thus the negative evidence all goes in favour of the radio-activity of ordinary materials. Since we can discover no other source of the radiation emitted by the metals we conclude that the metals must themselves be radiating. Such an argument is, however, always weak, and is by itself somewhat unconvincing. Fortunately there is some, though it must be admitted rather scanty, evidence which tends to confirm this conclusion.

The problem of ascertaining whether there is any radiation emitted by ordinary metals when due allowance has been made for the presence of radioactive impurities, secondary radiation, and other disturbing causes, has been attacked by Mr. N. R. Campbell,' whose experiments seem to show that part of the ionization inside a closed vessel is due to rays emitted by the walls of the enclosure. The absorption of this radiation by air is comparable with that 1 N. R. Campbell, loc. cit.

for the a rays from radium, but differs somewhat for different materials. Assuming that this radiation is of the a type, it is shown by Campbell that the "ranges" of the rays emitted by different materials can be measured. When this is done it is found that the ranges of the rays from each metal have a definite value, as would be expected if the metals were truly radioactive. But it must be remembered that the effects to be measured are extremely small and the experiments difficult to perform. It would therefore be rash to accept without further evidence the conclusion that radioactivity is a universal property of matter.

There are already several known radioactive elements constantly emitting radiations and thereby being transformed into other substances. It is interesting to contemplate that this most remarkable property may not be confined to these special forms of matter but may be shared to a greater or less extent by everything we see around us.

CHAPTER XI

THE MECHANISM OF RADIOACTIVE CHANGES

NATURE OF RADIOACTIVE CHANGES

HITHERTO only the properties of radioactive bodies and the nature of the changes going on during their transformation have been considered. Little or nothing has been said as to the mechanism by which these changes. may be considered to be taking place. It has been shown that radioactive bodies are constantly giving out considerable quantities of energy during their transformations, and the question naturally arises as to the source of this energy. There are, broadly, two points of view from which the matter may be regarded. First, it may be supposed that the energy is derived from sources external to the radioactive bodies, and that by some property peculiar to these substances they are capable

of absorbing energy sufficient to account for their radio-activity. For this purpose it has been assumed that space is always being traversed by radiations unabsorbed by ordinary materials, but capable of being absorbed even by small quantities of the radioactive substances, and that consequently we are unaware of the existence of these radiations except by the intervention of radioactive bodies. Ingenious as this hypothesis appears, there is no evidence to support it; for though it seems certain that there is always traversing space a penetrating type of radiation, yet, as far as can be tested by experiment, this radiation has nothing whatever to do with the phenomena of radio-activity. In the first place, the energy which might be supposed to be communicated to a radioactive body in this way is too small to account for the heat emitted by it; and secondly, it is impossible to influence the activity, say of radium, however great be the precautions observed to screen it from external radiation. For these and other reasons it is difficult to reconcile the view that radio-activity is due to the