"The decay of the activity of polonium with time has led to the view that polonium is not a new active substance, but merely active bismuth, i.e. bismuth which in some way had been made active by admixture with radio-active bodies. The activity of any product is not necessarily a proof that a radio-element is present, for it has been shown that many inactive elements become active by association with active matter. 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."