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.

Let us see how many millicuries we will in future be able to turn out in the week in our new Dublin Radium Institute.[1] We shall have about 130 milligrammes of radium.  In 3.8 days we get 65 millicuries from this—­i.e. half the equilibrium amount of 130 millicuries.  Hence in the week, we shall have about 130 millicuries.

This is not much.  Many experts consider this little enough for one tube.  But here in Dublin we have been using the emanation in a more economical and effective manner than is the usage elsewhere; according to a method which has been worked out and developed in our own Radium Institute.  The economy is obtained by the very simple expedient of minutely subdividing the’ dose.  The system in vogue, generally, is to treat the tumour by inserting into it one or two very active