
290 EXCITED RADIO-ACTIVITY [CH. 
the bismuth did not decay over the period of examination, but 
observations were not made over sufficient lengths of time to make 
certain of this. There are two points of view that have been taken 
in regard to radio-active induction. Some have supposed that the 
inactive molecules of the substance themselves temporarily acquire 
the property of radio-activity, after admixture with a very active 
substance like radium or actinium. On this view the radio-active 
bismuth is in reality bismuth, some of the matter of which has 
temporarily acquired radio-active property. 
On the other point of view, production of activity in mactive 
bodies is either due to a slight admixture of the active element, 
or to a removal with the substance of a radio-active product 
of the element. In the former case, the activity of the body is 
permanent; in the latter, it decays with the time, according to the 
same law as the decay of activity of the separated product. For 
example, if barium is precipitated im an uranium solution, the 
barium is active, and its activity decays at the same rate as the 
separated Ur X. In fact, the barium precipitate carried down with 
it the matter Ur X. 
So far, however, no case has yet been observed when any body 
has acquired the property of radio-activity by exposure to the 
radiations alone of the radio-elements. The evidence at present 
supports the view that the activity produced in inactive bodies is 
due to a separation with it of an active product. The experiments 
of F. von Lerch, described in sections 178, 179, show that many 
metals are able to become active when placed in a solution of - 
the emanation X of thorium. This activity is due to a deposit 
of the emanation X on the metal. The activity is removable by 
precipitation and also in some cases by electrolysis. In the case of 
solutions obtained from pitchblende, it is thus not surprising that 
a similar action occurs, and that many substances possess some 
temporary activity at the time of their separation. One or more 
of the numerous active products in pitchblende is precipitated 
with the substance, and the activity then decays with the time. 
188. Possible origin of polonium. Mme Curie has not 
yet been able to purify polonium sufficiently to obtain any spectro- 
scopic evidence of a new element. Gjuesel has consistently taken 
