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f beta and gamma radio…activity in the solid newly deposited by the emanation； this rise measuring the growth of the first radio…active offspring of one of the non…active bodies。 Some of the radium products give out alpha…rays only； one beta… and gamma…rays； while one yields all three types of radiation。 The pedigree of the radium family may be expressed in the following table； the time noted in the second column being the time required for a given quantity to be half transformed into its next derivative。
Time of half Radio… Properties decay activity
Radium About 2600 years alpha rays Element chemically analogous to barium。 》 Emanation 3。8 days alpha rays Chemically inert gas； condenses at …150 deg C。 》 Radium…A 3 minutes alpha rays Behaves as a solid deposited on surfaces； concentrated on a negative electrode。 》 Radium…B 21 minutes no rays Soluble in strong acids； volatile at a white heat； more volatile than A or C。 》 Radium…C 28 minutes alpha； beta； Soluble in strong acids； less gamma rays volatile than B。 》 Radium…D about 40 years no rays Soluble in strong acids； volatile below 1000 deg C。 》 Radium…E 6 days beta； gamma Non…volatile at 1000 deg C。 rays 》 Radium…F 143 days alpha rays Volatile at 1000 deg C。 Deposited from solution on a bismuth plate。
Of these products； A； B； and C constitute that part of the active deposit of the emanation which suffers rapid decay and nearly disappears in a few hours。 Radium…D； continually producing its short…lived descendants E and F； remains for years on surfaces once exposed to the emanation； and makes delicate radio…active researches impossible in laboratories which have been contaminated by an escape of radium emanation。
A somewhat similar pedigree has been made out in the case of thorium。 Here thorium…X is interposed between thorium and its short…lived emanation； which decays to half its initial quantity in 54 seconds。 Two active deposits； thorium A and B； arise successively from the emanation。 In uranium； we have the one obvious derivative uranium…X； and the question remains whether this one descent can be connected with any other individual or family。 Uranium is long…lived； and emits only alpha…rays。 Uranium…X decays to half value in 22 days； giving out beta… and gamma…rays。 Since our evidence goes to show that radio…activity is generally accompanied by the production of new elements； it is natural to search for the substance of uranium…X in other forms； and perhaps under other names； rather than to surrender immediately our belief in the conservation of matter。
With this idea in mind we see at once the significance of the constitution of uranium minerals。 Formed in the remote antiquity of past geological ages； these minerals must become store…houses of all the products of uranium except those which may have escaped as gases or possibly liquids。 Even gases may be expected to some extent to be retained by occlusion。 Among the contents of uranium minerals； then； we may look for the descendants of the parent uranium。 If the descendants are permanent or more long…lived than uranium； they will accumulate continually。 If they are short…lived； they will accumulate at a steady rate till enough is formed for the quantity disintegrating to be equal to the quantity developed。 A state of mobile equilibrium will then be reached； and the amount of the product will remain constant。 This constant amount of substance will depend only on the amount of uranium which is its source； and； for different minerals； if all the product is retained； the quantity of the product will be proportional to the quantity of uranium。 In a series of analyses of uranium minerals； therefore； we ought to be able to pick out its more short…lived descendants by seeking for instances of such proportionality。
Now radium itself is a constituent of uranium minerals； and two series of experiments by R。J。 Strutt and B。B。 Boltwood have shown that the content of radium； as measured by the radio…activity of the emanation； is directly proportional to the content of uranium。 （Strutt； 〃Proc。 Roy。 Soc。〃 A； February 1905； Boltwood； 〃Phil。 Mag。〃 April； 1905。） In Boltwood's investigation； some twenty minerals； with amounts of uranium varying from that in a specimen of uraninite with 74。65 per cent。； to that in a monazite with 0。30 per cent。； gave a ratio of uranium to radium； constant within about one part in ten。
The conclusion is irresistible that radium is a descendant of uranium； though whether uranium is its parent or a more remote ancestor requires further investigation by the radio…active genealogist。 On the hypothesis of direct parentage； it is easy to calculate that the amount of radium produced in a month by a kilogramme of a uranium salt would be enough to be detected easily by the radio…activity of its emanation。 The investigation has been attempted by several observers； and the results； especially those of a careful experiment of Boltwood； show that from purified uranium salts the growth of radium； if appreciable at all； is much less than would be found if the radium was the first product of change of the uranium。 It is necessary； therefore； to look for one or more intermediate substances。
While working in 1899 with the uranium residues used by M。 and Mme Curie for the preparation of radium； Debierne discovered and partially separated another radio…active element which he called actinium。 It gives rise to an intermediate product actinium…X； which yields an emanation with the short half…life of 3。9 seconds。 The emanation deposits two successive disintegration products actinium…A and actinium…B。
Evidence gradually accumulated that the amounts of actinium in radio…active minerals were； roughly at any rate； proportional to the amounts of uranium。 This result pointed to a lineal connection between them； and led Boltwood to undertake a direct attack on the problem。 Separating a quantity of actinium from a kilogramme of ore； Boltwood observed a growth of 8。5 x （10 to the power …9） gramme of radium in 193 days； agreeing with that indicated by theory within the limits of experimental error。 （〃American Journal of Science〃； December； 1906。） We may therefore insert provisionally actinium and its series of derivatives between uranium and radium in the radio… active pedigree。
Turning to the other end of the radium series we are led to ask what becomes of radium…F when in turn it disintegrates？ What is the final non… active product of the series of changes we have traced from uranium through actinium and radium？
One such product has been indicated above。 The alpha…ray particles appear to possess the mass of h