8 RADIO-ACTIVE SUBSTANCES [cH. 
of small particles projected at great velocities. The absence of the 
ordinary properties of transverse light waves is thus to be expected. 
7. The rays from uranium are complex in character, and, in 
addition to the penetrating deviable rays, there is also given off 
a radiation very readily absorbed by passing through thin layers 
of metal foil, or by traversing a few centimetres of air. The 
photographic action due to these rays is very feeble in comparison 
with that of the penetrating rays, although the discharge of 
electrified bodies is mainly caused by them. Besides these two 
types of rays, some rays are emitted which are of an extremely 
penetrating character and are non-deviable by a magnetic field. 
These rays are difficult to detect photographically, but can be 
readily examined by the electric method. 
8. The question naturally arose whether the property of 
spontaneously giving out penetrating radiations was confined to 
uranium and its compounds, or whether it was exhibited to any 
appreciable extent by other substances. 
By the electrical method, with an electrometer of ordinary 
sensitiveness, any body which possesses an activity of the order of 
1/100 of that of uranium can be detected. With an electroscope of 
special construction, such as has been designed by C. T. R. Wilson 
for his experiments on the natural ionization of air, a substance 
of activity 1/10000 and probably 1/100000 of that of uranium can 
be detected. 
If an active body like uranium be mixed with an inactive body, 
the resulting activity in the mixture is generally considerably less 
than that due to the active substance alone. This is due to the 
absorption of the radiation by the inactive matter present. The 
amount of decrease largely depends on the thickness of the layer - 
from which the activity is determined. 
Mme Curie made a detailed examination by the electrical 
method of the great majority of known substances, including the 
very rare elements, to see if they possessed any activity. In cases 
when it was possible, several compounds of the elements were 
examined. With the exception of thorium and phosphorus, none 
of the other substances possessed ‘an activity even of the order of 
1/100 of uranium. | 
