1] RADIO-ACTIVE SUBSTANCES 21 
The activity of these substances, when removed from the active 
element, is however only transient, and decays gradually with the 
time. This activity is not due to the presence of the radio-element 
itself. For example, barium separated from radium is strongly 
active, although the spectroscopic examination shows no trace 
of the radium lines. 
In order to explain this temporary activity m inactive matter 
it has been supposed that the non-active matter is made active by 
“induction” during its mixture with the active material. The 
underlying idea has been that inactive bodies themselves acquire 
the property of radio-activity. There is no evidence however that 
such is the case. The evidence rather points to the conclusion 
that the activity is due, not to any alteration of the mactive body 
itself, but to an admixture with it of a very small quantity of 
intensely active matter. The active matter that causes this so- 
called “induced” activity 1s itself a product of the disintegration of 
the radio-element and differs from it in chemical properties. 
The subject is a complicated one, and it cannot be discussed with 
advantage at this stage; it will, however, be considered in detail 
in section 187. On the above view the active bismuth contains 
a small quantity of matter, which weight for weight is probably 
far more active than radium, but the activity of which decays 
with time. The active matter is allied in chemical properties to 
bismuth, but possesses some distinct analytical properties which 
allow of a partial separation. The absence of any new lines in the 
spectrum is to be expected if, even in the most active bismuth 
prepared, the active matter exists in very small quantity. 
19. The discussion of the nature of polonium was renewed by 
the discovery of Marckwald? that a substance similar to polonium, 
of which the activity did not decay with time, could be separated 
from pitchblende. The method of separation from the bismuth 
chloride solution obtained from uranium residues was very simple. 
A rod of bismuth, dipped in the active solution, rapidly became 
coated with a black deposit, which was intensely active. This 
deposit was continued until the whole of the activity was removed 
from the solution. From 850 grammes of bismuth solution, 
1 Ber. deutsch. chem. Ges., p. 2285, 1902 ; Phys. Zeit., No. 1b, p. 51, 1902. 
