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which the names of uranium…X and thorium…X have been given。 These bodies behave differently from their parents uranium and thorium； and show all the signs of distinct chemical individuality。 They are strongly radio…active； while； after the separation； the parents uranium and thorium are found to have lost some of their radio…activity。 If the X…substances be kept； their radio…activity decays； while that of the uranium or thorium from which they were obtained gradually rises to the initial value it had before the separation。 At any moment； the sum of the radio…activity is constant； the activity lost by the product being equal to that gained by the parent substance。 These phenomena are explained if we suppose that the X…product is slowly produced in the substance of the parent； and decays at a constant rate。 Uranium； as usually seen； contains a certain amount of uranium…X； and its radio…activity consists of two partsthat of the uranium itself； and that of the X product。 When the latter is separated by means of its chemical reactions； its radio…activity is separated also； and the rates of decay and recovery may be examined。
Radium and thorium； but not uranium； give rise to radio…active gases which have been called emanations。 Rutherford has shown that their radio… activity； like that of the X products； suffers decay； while the walls of the vessel in which the emanation is confined； become themselves radio… active。 If washed with certain acids； however； the walls lose their activity； which is transferred to the acid； and can be deposited by evaporation from it on to a solid surface。 Here again it is clear that the emanation gives rise to a radio…active substance which clings to the walls of the vessel； and is soluble in certain liquids； but not in others。
We shall return to this point； and trace farther the history of the radio… active matter。 At present we wish to emphasise the fact that； as in other cases； the radio…activity of the emanation is accompanied by the appearance of a new kind of substance with distinct chemical properties。
We are now in a position to consider as a whole the evidence on the question of the source of radio…active energy。
（1） Radio…activity is accompanied by the appearance of new chemical substances。 The energy liberated is therefore probably due to the associated chemical change。 （2） The activity of a series of compounds is found to accompany the presence of a radio…active element； the activity of each compound depends only on the contents of the element； and is independent of the nature of its combination。 Thus radio…activity is a property of the element； and is not affected by its state of isolation or chemical combination。 （3） The radio…activity of a simple transient product decays in a geometrical progression； the loss per second being proportional to the mass of substance still left at the moment； and independent of its state of concentration or dilution。 This type of reaction is well known in chemistry to mark a mono…molecular change； where each molecule is dissociated or altered in structure independently。 If two or more molecules were concerned simultaneously； the rate of reaction would depend on the nearness of the molecules to each other； that is； to the concentration of the material。 （4） The amount of energy liberated by the change of a given mass of material far transcends the amount set free by any known ordinary chemical action。 The activity of radium decays so slowly that it would not sink to half its initial value in less than some two thousand years； and yet one gramme of radium emits about 100 calories of heat during each hour of its existence。
The energy of radio…activity is due to chemical change； but clearly to no chemical change hitherto familiar to science。 It is an atomic property； characteristic of a given element； and the atoms undergo the change individually； not by means of interaction among each other。 The conclusion is irresistible that we are dealing with a fundamental change in the structure of the individual atoms； which； one by one； are dissociating into simpler parts。 We are watching the disintegration of the 〃atoms〃 of the chemist； hitherto believed indestructible and eternal； and measuring the liberation of some of the long…suspected store of internal atomic energy。 We have stumbled on the transmutation dreamed by the alchemist； and discovered the process of a veritable evolution of matter。
The transmutation theory of radio…activity was formulated by Rutherford （Rutherford； 〃Radio…activity〃 （2nd edition）； Cambridge； 1905； page 307。） and Soddy in 1903。 By its light； all recent work on the subject has been guided； it has stood the supreme test of a hypothesis； and shown power to suggest new investigations and to co…ordinate and explain them； when carried out。 We have summarised the evidence which led to the conception of the theory； we have now to consider the progress which has been made in tracing the successive disintegration of radio…active atoms。
Soon after the statement of the transmutation theory； a striking verification of one of its consequences appeared。 The measurement of the magnetic and electric deflection of the alpha…rays suggested to Rutherford the idea that the stream of projectiles of which they consisted was a flight of helium atoms。 Ramsay and Soddy； confining a minute bubble of radium emanation in a fine glass tube； were able to watch the development of the helium spectrum as； day by day； the emanation decayed。 By isolating a very narrow pencil of alpha…rays； and watching through a microscope their impact on a fluorescent screen； Rutherford has lately counted the individual alpha…projectiles； and confirmed his original conclusion that their mass corresponded to that of helium atoms and their charge to double that on a univalent atom。 （〃Proc。 Roy。 Soc。〃 A； page 141； 1908。） Still more recently； he has collected the alpha…particles shot through an extremely thin wall of glass； and demonstrated by direct spectroscopic evidence the presence of helium。 （〃Phil。 Mag。〃 February 1909。）
But the most thorough investigation of a radio…active pedigree is found in Rutherford's classical researches on the successive disintegration products of radium； in order to follow the evidence on which his results are founded； we must describe more fully the process of decay of the activity of a simple radio…active substance。 The decay of activity of the body known as uranium…X is shown in a falling curve （Fig。 1。）。 It will be seen that； in each successive 22 days； the activity falls to half the value it possessed at the beginning。
This change in a geometrical progression is characteristic of simple radio… active processes； and can be expressed mathematically by a simple exponential formula。
As we have said above； solid bodies exposed to the emanations of radium or thorium become coated with a radio…active deposit。 The rate of decay of this activity depends on the time of exposure to the emanation； and does not always show the usual simple type of curve。 Thus the activity of a rod exposed to radium emanation for 1 minute decays in accordance with a curve （Fig。 2） which represents the activity as measured by the alpha…rays。 If the electroscope be screened from the alpha…rays； it is found that the activity of the rod in beta… an gamma…rays increases for some 35 minutes and then diminishes （Fig。 3。）。
These complicated relations have been explained satisfactorily and completely by Rutherford on the hypothesis of successive changes of the radio…active matter into one new body after another。 （Rutherford； 〃Radio… activity〃 （2nd edition）； Cambridge； 1905； page 379。） The experimental curve represents the resultant activity of all the matter present at a given moment； and the process of disentangling the component effects consists in finding a number of curves； which express the rise and fall of activity of each kind of matter as it is produced and decays； and； fitted together； give the curve of the experiments。
Other methods of investigation also are open。 They have enabled Rutherford to complete the life…history of radium and its products； and to clear up doubtful points left by the analysis of the curves。 By the removal of the emanation； the activity of radium itself has been shown to consist solely of alpha…rays。 This removal can be effected by passing air through the solution of a radium salt。 The emanation comes away； and the activity of the deposit which it leaves behind decays rapidly to a small fraction of its initial value。 Again； some of the active deposits of the emanation are more volatile than others； and can be separated from them by the agency of heat。
From such evidence Rutherford has traced a long series of disintegration products of radium； all but the first of which exist in much too minute quantities to be detected otherwise than by their radio…activities。 Moreover； two of these products are not themselves appreciably radio… active； though they are born from radio…active parents； and give rise to a series of radio…active descendants。 Their presence is inferred from such evidence as the rise of beta and gamma radio…activity in the solid newly deposited by the emanation； this rise measuring t