dium
C. One thing here we must be clear about. With a given quantity
of Radium only a certain definitely limited amount of Radium C,
or of emanation, or any other of the derived bodies, will be
associated. Why is this? The answer is because the several
successive elements are themselves decaying --_i.e._ changing one
into the other. The atomic per-

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centage of each, which decays in a second, is a fixed quantity
which we cannot alter. Now if we picture radium which has been
completely deprived of its emanation, again accumulating by
automatic transmutation a fresh store of this element, we have to
remember:-- (i) That the rate of creation of emanation by the
radium is practically constant; and (2) that the absolute amount
of the emanation decaying per second increases as the stock of
emanation increases. Finally, when the amount of accumulated
emanation has increased to such an extent that the number of
emanation atoms transmuting per second becomes exactly equal to
the number being generated per second, the amount of emanation
present cannot increase. This is called the equilibrium amount.
If fifteen members are elected steadily each year into a
newly-founded society the number of members will increase for the
first few years; finally, when the losses by death of the members
equal about fifteen per annum the society can get no bigger. It
has attained the equilibrium number of members.

This applies to every one of the successive elements. It takes
twenty-one days for the equilibrium quantity of emanation to be
formed in radium which has been completely de-emanated; and it
takes 3.8 days for half the equilibrium amount to be formed.
Again, if we start with a stock of emanation it takes just three
hours for the equilibrium amount of Radium C to be formed.

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We can evidently grow Radium C either from radium itself or from
the emanation of radium. If we use a tube of radium we have an
almost perfectly constant quantity of Radium C present, for as
fast as the Radium C and intervening elements decay the Radium,
which only diminishes very slowly in amount, makes up the loss.
But, if we start off with a tube of emanation, we do not possess
a constant supply of Radium C, because the emanation is decaying
fairly rapidly and there is no radium to make good its loss. In
3.8 days about one half the emanation is transmuted and the
Radium C decreases proportionately and, of course, with the
Radium C the valuable rad