THE RATE OF CHANGE OF RADIUM 19 



annum, is so slow that over ordinary periods of 

 time it is imperceptible, and yet so rapid that the 

 amount of energy continuously evolved is, considering 

 the excessively minute quantity of matter, truly 

 astonishing. The other members of the series which 

 change more rapidly possess a radioactivity which, 

 though it is more intense, is more ephemeral. More- 

 over, the quantity of each member of the series, 

 coexisting with its parent, is proportional to its 

 period of life. For it is a balanced or equilibrium 

 quantity, when the rate of formation equals the rate 

 of change. The more quickly changing members 

 never accumulate in ponderable quantity and, for 

 them, it is impossible to prove, by the older methods 

 of science, that they are, indeed, new elementary 

 substances, possessing distinct chemical character, 

 atomic weight and spectrum. For radium, though 

 the proportion in which it exists in the richest 

 uranium mineral is exceedingly minute, it is just 

 possible to obtain enough to weigh and to prepare 

 in a pure condition for chemical examination. 



The most slowly changing members, on the other 

 hand, are the parent-elements, uranium and thorium, 

 which were well studied by chemists for a century, so 

 feeble is their radioactivity and so slow their rate of 

 disintegration, without a suspicion that, in them, 

 the oft-suspected process of the evolution of the 

 elements was still in progress before their eyes. 



There is a certain quality of permanence about 

 experimental scientific discovery which is not always 

 believed. An important addition to experimental 

 knowledge, whether made in the time of Robert 

 Boyle or yesterday, is never displaced. Points of 

 view may change, theories interpreting and explain- 

 ing experimental knowledge may have their periods 

 of adolescence, maturity and decline, but the frame- 

 work of the structure, the experimental fact round 



