ties of the two sets of radiation if they are essentially similar in nature. There is perhaps a still more serious difficulty in the way of accepting this theory. If they rays convey with them an electric charge, then it should be possible to detect this charge directly just as in the case of the B rays. All attempts have, however, hitherto failed to reveal this effect, although some experiments by Paschen1 seemed at first to suggest that there was such a charge carried by the rays. But it has since been shown by Eve2 and others that at points where the y rays strike a solid obstacle, secondary rays of the ẞ type are set up, and it is possible to trace the charges communicated to bodies upon which the y rays fall to this secondary radiation. A second hypothesis to account for the properties of the y rays is that they consist of a very penetrating type of Röntgen rays. We have already seen that the Röntgen rays carry no charge with them, and they cannot 1 Paschen, Ann. der Physik, Nos. 6 and 7, 1904. be deflected in a magnetic field; they probably consist of some kind of disturbance propagated through the ether with the velocity of light. Some recent measurements by Marx' confirm this view, for he succeeded in showing that they do, in fact, travel with this velocity. If the y rays were of this nature their behaviour would be explained, and on the whole the balance of evidence supports this view rather than the corpuscular hypothesis. The discussion as to the nature of Röntgen and y rays has quite recently been revived by Bragg in a paper in the Philosophical Magazine (October, 1907). The view is there expressed that these rays may consist of uncharged material particles formed by the combination of an a and B particle. It is shown that such a doublet would exhibit the properties of the y rays. 1 Marx, Annalen der Physik, Vol. XX, p. 677. 1906. CHAPTER V PHYSICAL AND CHEMICAL EFFECTS OF THE RADIATIONS FROM RADIOACTIVE BODIES THE most important properties of the various radiations emitted by radioactive bodies have been carefully studied in the last chapter. There still remain to be considered certain physical and chemical effects which are produced when the radiations from a radioactive substance are allowed to fall bodies. upon certain FLUORESCENCE PRODUCED BY RADIUM We have already seen that when the radiations from radium fall upon certain substances a brilliant fluorescence is produced. Thus if a screen of zinc sulphide is interposed in the path of the rays, the screen can be seen in the dark on account of the fluorescence which has been induced in it. This action has already been studied, and it has been shown that at any rate when the fluorescence is due to the a rays the effect may be resolved by means of the spinthariscope into a large number of minute flashes produced whenever an a particle strikes the zinc sulphide. The property is a fairly general one, and is not confined to zinc sulphide. Thus platinocyanides which we have seen fluoresce under the influence of the Röntgen rays may also be excited by the radiations emitted by radium. Furthermore, many crystals fluoresce brilliantly under the action of the radiations; thus, for example, the diamond and the ruby exhibit this property to a marked extent, as do crystals of fluorspar, and to a less extent certain kinds of glass. With this phenomenon of fluorescence there are often associated certain secondary effects in the bodies which are caused to fluoresce. Thus it is frequently noticed that when a body has been excited for some time to fluorescence by the action of the rays from radium its colour changes. For instance, if ordinary soda glass is exposed to the radiations from radium the glass gradually as sumes a beautiful violet colour. Similarly if potash glass is treated in the same way, the glass which previously was colourless is now found to possess a distinct brown coloration. It is on this account that the glass vessels used to contain radium salts or their solutions are nearly always found to be more or less highly coloured. By strongly heating the glass the coloration may be removed and the glass restored to its original condition. Similar fluorescent and colour effects can be advantageously observed and studied in the case of crystals of fluorspar. This substance occurs in nature in many different varieties. In some cases the crystals are found quite colourless and transparent; in others the crystals are of various tints, which may be blue, green, yellow, or violet. If such crystals are exposed to the action of the rays from radium they are seen to fluoresce brilliantly, and the fluorescence persists for a considerable time after the removal of the radium. addition to this action the colour of the crystal is frequently changed; thus a colourless crystal may become coloured, or the tint In |