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One such product has been indicated above。 The alpha…ray particles appear to possess the mass of helium atoms； and the growth of helium has been detected by its spectrum in a tube of radium emanation。 Moreover； helium is found occluded in most if not all radio…active minerals in amount which approaches； but never exceeds； the quantity suggested by theory。 We may safely regard such helium as formed by the accumulation of alpha…ray particles given out by successive radio…active changes。
In considering the nature of the residue left after the expulsion of the five alpha…particles； and the consequent passage of radium to radium…F we are faced by the fact that lead is a general constituent of uranium minerals。 Five alpha…particles； each of atomic weight 4； taken from the atomic weight （about 225） of radium gives 205a number agreeing fairly well with the 207 of lead。 Since lead is more permanent than uranium； it must steadily accumulate； no radio…active equilibrium will be reached； and the amount of lead will depend on the age of the mineral as well as on the quantity of uranium present in it。 In primary minerals from the same locality； Boltwood has shown that the contents of lead are proportional to the amounts of uranium； while； accepting this theory； the age of minerals with a given content of uranium may be calculated from the amount of lead they contain。 The results vary from 400 to 2000 million years。 （〃American Journal of Science〃； October； 1905； and February； 1907。）
We can now exhibit in tabular form the amazing pedigree of radio…active change shown by this one family of elements。 An immediate descent is indicated by 》； while one which may either be immediate or involve an intermediate step is shown by 。。。。 No place is found in this pedigree for thorium and its derivatives。 They seem to form a separate and independent radio…active family。
Atomic Weight Time of half Radio…Activity decay
Uranium 238。5 alpha 》 Uranium…X ？ 22 days beta； gamma 。。。 Actinium ？ ？ no rays 》 Actinium…X ？ 10。2 days alpha （beta； gamma） 》 Actinium Emanation ？ 3。9 seconds alpha 》 Actinium…A ？ 35。7 minutes no rays 》 Actinium…B ？ 2。15 minutes alpha； beta； gamma 。。。 Radium 225 about 2600 years alpha 》 Radium Emanation ？ 3。8 days alpha 》 Radium…A ？ 3 minutes alpha 》 Radium…B ？ 21 minutes no rays 》 Radium…C ？ 28 minutes alpha； beta； gamma 》 Radium…D ？ about 40 years no rays 》 Radium…E ？ 6 days beta （gamma） 》 Radium…F ？ 143 days alpha 。。。 Lead 207 ？ no rays
As soon as the transmutation theory of radio…activity was accepted； it became natural to speculate about the intimate structure of the radio… active atoms； and the mode in which they broke up with the liberation of some of their store of internal energy。 How could we imagine an atomic structure which would persist unchanged for long periods of time； and yet eventually spontaneously explode； as here an atom and there an atom reached a condition of instability？
The atomic theory of corpuscles or electrons fortunately was ready to be applied to this new problem。 Of the resulting speculations the most detailed and suggestive is that of J。J。 Thomson。 （〃Phil。 Mag。〃 March； 1904。） Thomson regards the atom as composed of a number of mutually repelling negative corpuscles or electrons held together by some central attractive force which he represents by supposing them immersed in a uniform sphere of positive electricity。 Under the action of the two forces； the electrons space themselves in symmetrical patterns； which depend on the number of electrons。 Three place themselves at the corner of an equilateral triangle； four at those of a square； and five form a pentagon。 With six； however； the single ring becomes unstable； one corpuscle moves to the middle and five lie round it。 But if we imagine the system rapidly to rotate； the centrifugal force would enable the six corpuscles to remain in a single ring。 Thus internal kinetic energy would maintain a configuration which would become unstable as the energy drained away。 Now in a system of electrons； electromagnetic radiation would result in a loss of energy； and at one point of instability we might well have a sudden spontaneous redistribution of the constituents； taking place with an explosive violence； and accompanied by the ejection of a corpuscle as a beta…ray； or of a large fragment of the atom as an alpha…ray。
The discovery of the new property of radio…activity in a small number of chemical elements led physicists to ask whether the property might not be found in other elements； though in a much less striking form。 Are ordinary materials slightly radio…active？ Does the feeble electric conductivity always observed in the air contained within the walls of an electroscope depend on ionizing radiations from the material of the walls themselves？ The question is very difficult； owing to the wide distribution of slight traces of radium。 Contact with radium emanation results in a deposit of the fatal radium…D； which in 40 years is but half removed。 Is the 〃natural〃 leak of a brass electroscope due to an intrinsic radio…activity of brass； or to traces of a radio…active impurity on its surface？ Long and laborious researches have succeeded in establishing the existence of slight intrinsic radio…activity in a few metals such as potassium； and have left the wider problem still unsolved。
It should be noted； however； that； even if ordinary elements are not radio… active； they may still be undergoing spontaneous disintegration。 The detection of ray…less changes by Rutherford； when those changes are interposed between two radio…active transformations which can be followed； show that spontaneous transmutation is possible without measureable radio… activity。 And； indeed； any theory of disintegration； such as Thomson's corpuscular hypothesis； would suggest that atomic rearrangements are of much more general occurrence than would be apparent to one who could observe them only by the effect of the projectiles； which； in special cases； owing to some peculiarity of atomic configuration； happened to be shot out with the enormous velocity needed to ionize the surrounding gas。 No evidence for such ray…less changes in ordinary elements is yet known； perhaps none may ever be obtained； but the possibility should not be forgotten。
In the strict sense of the word； the process of atomic disintegration revealed to us by the new science of radio…activity can hardly be called evolution。 In each case radio…active change involves the breaking up of a heavier； more complex atom into lighter and simpler fragments。 Are we to regard this process as characteristic of the tendencies in accord with which the un