magnet at right angles to the lines of magnetic force, and to fall on a phosphorescent screen. A spot of light may be seen on the screen in the dark, and on exciting the magnet the spot of light moves over the screen and the deflection is such as would be produced on negatively charged particle moving from the radium to the screen. This shows that at least some of the rays from radium consist of negatively charged particles, the magnitude of the deflection being such as to indicate at the same time that they are travelling with exceedingly high velocities. These rays are fairly penetrating, and are the B rays mentioned above.

Although some of the rays are easily deviable by a magnet, yet it was shown by P. Curie that a portion of the rays passed through undeviated, and that it was precisely these rays which are most effective in producing ionization and which are, in fact, the easily absorbed a rays. A very careful investigation of these rays by Rutherford subsequently revealed the fact that they can be deflected by very intense magnetic fields and that they consist of positively charged particles projected with

high velocities. The experiment is, however, difficult to perform, and it should be fully understood that the deflection of the a rays by a magnetic field is very much less than for the B rays.

They rays have so far resisted all attempts to deflect them in a magnetic field, and seem, therefore, to be uncharged, and it seems almost certain that they consist of ether vibrations of some kind and are similar in nature to the Röntgen rays.1 They have been shown by Marx to travel with the same velocity as light.

The radiations from a radioactive substance may thus be complex, consisting of at most three types of radiation. If the radiation from radium is examined it is found to consist of these three types of radiation; but it does not follow that all radioactive bodies give out all three types. For instance, the rays from polonium consist exclusively of a rays, there being no penetrating radiation present, so that the a rays may exist without ẞ or y rays. In all known cases, however, ẞ and y rays are found to occur together.

1 But see note on p. 124.

THE ALPHA RAYS

MAGNETIC AND ELECTRIC DEFLECTION OF THE a RAYS

It is only with considerable difficulty that the a rays can be deflected, and it was some time after the discovery of this easily absorbed radiation that Rutherford1 at last succeeded

in deflecting them and demonstrating their true nature. The experiment was performed in the following manner.

The rays from a thin layer of uncovered radium R (Fig. 10) passed up through a number of narrow slits and then through

1 Rutherford, Phil. Mag., February, 1903.

a thin aluminium leaf into a closed metal chamber C in which the ionization could be measured by means of a delicate gold-leaf electroscope not shown in the diagram. The slits through which the rays passed were situated in the field of a strong electromagnet, the lines of force being at right angles to the plane of the paper. If the rays were deflected by the magnetic field, in their passage through the slits, they would be deflected so as to strike the sides of the slits and be absorbed instead of passing into the chamber C. The rate at which the gold-leaf electroscope lost its charge should thus be reduced if the magnet produced any sensible deflection on the rays. With very intense magnetic fields this was, in fact, found to be the case.

Several precautions were necessary in carrying out the experiment. In the first place it is necessary to prevent the emanation1 from diffusing into C, for this gas is always given off from radium and is exceedingly radioactive. To prevent this a stream of hydrogen 1 See p. 51.

was kept passing downwards through the slits as indicated by the arrows in the diagram. If this precaution is omitted the effect of the direct radiation from the radium may easily be completely masked. In the second place it is necessary to make allowance for the B and y radiation, which passes through the slits and produces ionization independently of the a radiation when there is no magnetic field. For this purpose the radium was covered with a piece of mica, thick enough to absorb all the a radiation, but thin enough to allow most of the ẞ and y rays to pass through unabsorbed. In this way the effect due to these rays alone could be measured and allowed for. Taking these precautions Rutherford was able to deflect nearly the whole of the a radiation proceeding from the radium.

It now remained to find out in which direction the rays were deflected, and whether the deflection corresponded to a positive or negative charge carried by the a particles. To do this the slits described in the last experiment were replaced by a set of slits of