18 RADIO-ACTIVE SUBSTANCES [CH. 
by Mme Curie agrees with that required by the periodic system, 
it is advisable in the present state of our knowledge to accept the 
experimental number rather than the one deduced by Runge and > 
Precht from spectroscopic evidence. 
There is no doubt that radium is a new element possessing 
remarkable physical properties. The detection and separation of 
this substance, existing im such minute proportions in pitchblende, 
has been due entirely to the characteristic property we are con- 
sidering, and is the first notable triumph of the study of radio- 
activity. As we shall see later in chapter Vu, the property of radio- 
activity can be used, not only as a means of chemical research, but 
also as an extraordinarily delicate method of detecting chemical 
changes of a very special kind. 
15. Radiations from radium. On account of its enormous 
activity the radiations from radium are very intense: a screen 
of zinc sulphide, brought near a few centigrams of radium 
bromide, is lighted up quite brightly in a dark room, while 
brilhant fluorescence is produced on a screen of platimo-barium 
cyanide. An electroscope brought near is almost instantly 
discharged, while a photographic plate is immediately affected. 
At a distance of one metre, a day’s exposure to the radium 
rays would produce a strong impression. The radiations from 
radium are analogous to those of uranium, and consist of the three 
types of rays: easily absorbed, penetrating, and very penetrating. 
Radium also gives rise to an emanation similar to that of thorium, 
but with a very much slower rate of decay. The radium emanation 
retains its activity for several weeks, while that of thorium lasts 
only a few minutes. The emanation obtained from a few centi- 
erams of radium illuminates a screen of zinc sulphide with 
great briliancy. The very penetrating rays of radium are able to 
light up an X ray screen in a dark room, after passage through 
several centimetres of lead and several inches of iron. 
As in the case of uranium or thorium, the photographic action 
is mainly due to the penetrating or cathodic frays. The radio- 
graphs obtained with radium are very similar to those obtained 
with X rays, but lack the sharpness and detail of the latter. The 
rays are unequally absorbed by different kinds of matter, the 
