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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 radiations also decrease. In another 3.8
days—that is in about a week from the start—the radioactive value
of the tube has fallen to one-fourth of its original value.
But in spite of the inconstant character of the emanation tube
there are many reasons for preferring its use to the use of the
radium tube. Chief of these is the fact that we can keep the
precious radium safely locked up in the laboratory and not
exposed to the thousand-and-one risks of the hospital. Then,
secondly, the emanation, being a gas, is very convenient for
subdivision into a large number of very small tubes according to
the dosage required.
In fact the volume of the emanation is exceedingly minute. The
amount of emanation in equilibrium with one gramme of radium is
called the curie, and with one
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milligramme the millicurie. Now, the volume of the curie is only
a little more than one half a cubic millimetre. Hence in dealing
with emanation from twenty or forty milligrammes of radium we are
dealing with very small volumes.
How may the emanation be obtained? The process is an easy one in
skilled and practised hands. The salt of radium—generally the
bromide or chloride—is brought into acid solution. This causes
the emanation to be freely given off as fast as it is formed. At
intervals we pump it off with a mercury pump.