The values of the atomic weight and the logarithm of atomic weight for radium are put in brackets as they have been calculated by extrapolation of the straight line (Fig. 9).

Marshall-Watts1 has employed a different spectroscopic method of determining the atomic weight of radium, from which he deduces the value 225; Runge and Precht,2 however, criticize the validity of his method, and it is doubtful whether much weight can be given to this value.

It will thus be noted that there is a discrepancy between the value 226 of the atomic weight of radium obtained by direct chemical analysis and that of 258 obtained by the spectroscopic method of Runge and Precht. This discrepancy still awaits explanation. In the meantime, however, it would seem best to accept the value obtained by direct chemical analysis rather than to depend upon indirect spectroscopic evidence, the interpretation of which is doubtful.

1 Marshall-Watts, Phil. Mag. (6), Vol. V, p. 203 (1903); Vol. VI, p. 64 (1903).

2

Runge and Precht, Phil. Mag. (6), Vol. VI, p. 698, (1903).

CHAPTER IV

THE RADIATIONS EMITTED BY RADIOACTIVE

BODIES

THE α, ß, AND Y RAYS

BODIES which spontaneously give out radiations with definite photographic and electrical actions have been called radioactive, and, in fact, it is only thanks to this property that certain elements such as radium have been detected and isolated at all. Now the radiations emitted by different radioactive elements are not exactly similar, nor are the rays given out by one and the same body in general of one kind; on the contrary, they are often exceedingly complex. That this is the case may be easily shown.

If, for instance,

some radium be brought into the neighbourhood of a charged gold-leaf electroscope it will be discharged at a definite rate so long as the relative position of the radium and electro

scope is unaltered. If now an exceedingly thin film of solid matter such as a piece of aluminium leaf is interposed between the radium and the electroscope, the latter is still discharged, but at a somewhat slower rate. It is therefore evident that the rays coming from the radium can penetrate solid objects if sufficiently thin with only a slight diminution of intensity. If now more and more layers of aluminium leaf be interposed until the thickness of the aluminium has reached *005 millimetres, it is found that the intensity of radiation has been cut down to half its original value. The addition of a further layer of 005 millimetres of aluminium cuts down the radiation again to half, and so on for successive layers. If, however, this process is continued, it is found that the radiation reaching the electroscope from the radium cannot be completely intercepted, and after a certain thickness of aluminium has been interposed further layers of aluminium have little or no effect until the thickness has been very considerably increased. When the thickness reaches about 1 millimetre the radiation

begins again to be appreciably decreased, and at 5 millimetres is cut down to half. On proceeding still further a similar sequence of phenomena occurs, the radiation ultimately reaching a third constant level. In order to reduce this, it is necessary to interpose even greater thicknesses of aluminium, 8 centimetres being now required to cut down the radiation to half. It thus appears that the radiations are divided into three natural groups: (1) Those which are easily absorbed and which have been called a rays; (2) those which are less easily absorbed and have been called B rays; and (3) a very penetrating kind of radiation which is known as the y radiation.

This grouping of the rays into three distinct classes is of great importance, since a further study reveals the fact that the different kinds of rays exhibit important differences besides the dissimilarity in their penetrating power.

Some experiments which are easy to repeat were made by Giesel on the effect of a magnetic field on the radiation from radium. A thin pencil of rays from radium is allowed to pass between the pole pieces of an electro