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Period in whioh 1/2 element is transformed.

URANIUM 1 & 2 { a 6 } x 109 years.

URANIUM X { a ss } 24.6 days.

IONIUM { a 8 } x 104 years.

RADIUM { a ss } 2 x 102 years.

EMANATION { a } 8.85 days.

RADIUM A { a 8 } minutes.

RADIUM B { ss y } 26.7 minutes.

RADIUM C { a ss y } 13.5 minutes.

RADIUM D { ss } 15 years.

RADIUM E { ss y } 4.8 days.

RADIUM (Polonium) F { a } 140 days.

Table showing the successive generations of the elements of the Uranium-radium family, the character of their radiations and their longevity.

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the ss-rays emitted by radium are very “soft”—­i.e. slow and easily absorbed.  The a-ray is in no case available for more than mere surface application.  Hence we see that, contrary to what is generally believed, radium itself is of little direct therapeutic value.  Nor is the next body in succession—­the emanation, for it gives only a-rays.  In fact, to be brief, it is not till we come to Radium B that ss-rays of a relatively high penetrative quality are reached; and it is not till we come to Radium C that highly penetrative y-rays are obtained.

It is around this element, Radium C, that the chief medical importance of radioactive treatment by this family of radioactive bodies centres.  Not only are ss-rays of Radium C very penetrating, but the y-rays are perhaps the most energetic rays of the, kind known.  Further in the list there is no very special medical interest.

Now, how can we get a supply of this valuable element Radium C?  We can obtain it from radium itself.  For even if radium has been deprived of its emanation (which is easily done by heating it or bringing it into solution) in a few weeks we get back the Radium 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.