It is seen that the hard rays show a much closer agreement than the soft rays with the density law found for the γ rays. The high values previously obtained for the vapours of chloroform and carbon tetrachloride are greatly reduced, and are very nearly the same as for the γ rays. On the other hand, the vapour of methyl iodide is an exception, and still shows a high conductivity. The γ rays were, however, forty times as penetrating as the hard X rays, and it is probable that the value of methyl iodide would be reduced with still more penetrating X rays.
Relative conductivities of gases.
| Gas | Relative Density | “Soft” X rays | “Hard” X rays | γ rays |
|---|---|---|---|---|
| Hydrogen | ·07 | ·11 | ·42 | ·19 |
| Air | 1·0 | 1·0 | 1·0 | 1·0 |
| Sulphuretted Hydrogen | 1·2 | 6 | ·9 | 1·23 |
| Chloroform | 4·3 | 32 | 4·6 | 4·8 |
| Methyl Iodide | 5·0 | 72 | 13·5 | 5·6 |
| Carbon Tetrachloride | 5·3 | 45 | 4·9 | 5·2 |
The hard X rays were found to give far more secondary radiation than the γ rays, but this effect is probably also a function of the penetrating power of the primary rays. It will be seen later ([section 112]) that γ rays give rise to a secondary radiation of the β ray type. This has also been observed for the X rays.
Considering the experimental evidence as a whole, there is undoubtedly a very marked similarity between the properties of γ and X rays. The view that the γ rays are a type of very penetrating X rays, also receives support from theoretical considerations. We have seen ([section 52]) that the X rays are believed to be electromagnetic pulses, akin in some respects to short light waves, which are set up by the sudden stoppage of the cathode ray particles. Conversely, it is also to be expected that X rays will be produced at the sudden starting, as well as at the sudden stopping, of electrons. Since most of the β particles from radium are ejected from the radium atom with velocities much greater than the cathode particles in a vacuum tube, X rays of a very penetrating character will arise. But the strongest argument in support of this view is derived from an examination of the origin and connection of the β and γ rays from radio-active substances. It will be shown later that the α ray activity observed in radium arises from several disintegration products, stored up in the radium, while the β and γ rays arise only from one of these products named radium C. It is found, too, that the activity measured by the γ rays is always proportional to the activity measured by the β rays, although by separation of the products the activity of the latter may be made to undergo great variations in value.
Thus the intensity of the γ rays is always proportional to the rate of expulsion of β particles, and this result indicates that there is a close connection between the β and γ rays. Such a result is to be expected if the β particle is the parent of the γ ray, for the expulsion of each electron from radium will give rise to a narrow spherical pulse travelling from the point of disturbance with the velocity of light.
108. There is another possible hypothesis in regard to the nature of these rays. It has been shown (sections 48 and 82) that the apparent mass of an electron increases as the speed of light is approached; theoretically it should be very great when the velocity of the electron is exceedingly close to the velocity of light. In such a case, a moving electron would be difficult to deflect by a magnetic or electric field.
The view that the γ rays are electrons carrying a negative charge and moving with a velocity nearly equal to that of light has recently been advocated by Paschen[[173]]. He concluded from experiment that the γ rays like the β rays carried a negative charge. We have seen ([section 85]) that Seitz also observed that a small negative charge was communicated to bodies on which the γ rays impinged, but the magnitude of this charge was much smaller than that observed by Paschen. I do not think that much weight can be attached to observations that a small positive or negative charge is communicated to bodies on which the γ rays fall, for it will be shown later that a strong secondary radiation, consisting in part of electrons, is set up during the passage of the γ rays through matter. It is not improbable that the small charge observed is not a direct result of the charge carried by the γ rays, but is an indirect effect due to the secondary radiations emitted from the surface of bodies. There is no doubt that a thick lead vessel, completely enclosing a quantity of radium, acquires a small positive charge, but this result would follow whether the γ rays carry a charge or not, since the secondary radiations from the lead surface consist of projected particles which carry with them a negative charge.
On this corpuscular theory of the nature of the γ rays, each electron must have a large apparent mass, or otherwise it would be appreciably deflected by an intense magnetic field. The energy of motion of the electron must, in consequence, be very great, and, if the number of the electrons constituting the γ rays is of the same order of magnitude as the number of the β particles, a large heating effect is to be expected when the γ rays are stopped in matter. Paschen[[174]] made some experiments on the heat emission of radium due to the γ rays; he concluded that the γ rays were responsible for more than half of the total heat emission of radium and carried away energy at the rate of over 100 gram calories per hour per gram of radium. This result was not confirmed by later experiments of Rutherford and Barnes[[175]], who found that the heating effect of the γ rays could not be more than a few per cent. of the total heat emission of radium. These results will be considered later in [chapter XII].
The weight of evidence, both experimental and theoretical, at present supports the view that the γ rays are of the same nature as the X rays but of a more penetrating type. The theory that the X rays consist of non-periodic pulses in the ether, set up when the motion of electrons is arrested, has found most favour, although it is difficult to provide experimental tests to decide definitely the question. The strongest evidence in support of the wave nature of the X rays is derived from the experiments of Barkla[[176]], who found that the amount of secondary radiation set up by the X rays on striking a metallic surface depended on the orientation of the X ray bulb. The rays thus showed evidence of a one-sidedness or polarization which is only to be expected if the rays consist of a wave motion in the ether.