116 G. F. BECKER RELATIONS OF RADIOACTIVITY TO COSMOGONY 



number. Now the atomic weight of uranium is 238.5, the highest known, 

 while that of helium is 4 ; so that if « = 3, the atomic weight would be 

 226.5. Mrs Curie's latest determination of the atomic weight of radium 

 is 226.45 ; so that radium fits perfectly into the theoretical series. 



In uranium minerals radium, helium, and lead are always found 

 together, and Mr Boltwood ^ suggested that lead was probably the stable, 

 or relatively stable, end member of the series. If so, its atomic weight 

 should be 206.5. It is really 206.9 ; but this small discrepancy may well 

 be due to lack of precision in the atomic weight of helium, which is only 

 inferred from its density. Very possibly it may be found that the best 

 way of determining the atomic weight of helium is to divide the difference 

 between the atomic weights of uranium and lead by 8, which would give 

 He = 3.95. Lead has not yet been isolated from radium in appreciable 

 quantities, but this is probably only because the quantity of radium itself 

 available for experiment is almost infinitesimal. 



According to the theory, there should be two radioactive substances 

 yielding a particles intermediate between U and Ea and 4 between Ea 

 and Ph. Just four products capable of emitting a particles are already 

 kno\vn between Ea and Pb, but they have not been produced in sufficient 

 quantities and pure enough to permit atomic weight determinations. 

 Only one member between U and Ea has yet been discovered, and this, 

 called ionium, was detected during the summer of 1907 by Mr. Boltwood. 

 It is believed to be the immediate parent of radium or U — 2IIe. The 

 missing link is represented by "uranium X", which emits j8 particles 

 and y rays, but no a particles capable of detection by present methods. 

 It is considered probable that a particles may be given off from this sub- 

 stance, but at a velocity so low as not to ionize the gases through which 

 they pass. In that case they would fail to affect the electrometer, which 

 is the instrument employed to detect their presence. 



The several members of the uranium lead series emit a particles at 

 different characteristic velocities and their activities decay at different 

 characteristic rates. The law of decay of any one of them is assuredly 

 exponential, and observation tends to show that it is the simplest possible 

 exponential ; so that if I^ is the activity at time t, and J^ the activity at 

 the epoch from which time is counted, while A is a constant, 



_ ^« 



Here A. has a particular value for each radioactive substance. It follows 

 that when 



« = log 2/ A = 0.6932 /A. 



' Philosophical Magazine, vol. 9, 1905, p. 613. 



