THE MYSTERY OF RADIOACTIVITY 651 



independent judgment. The fact that there is so little criticism 

 has also much to do with the slowness with which the knowledge 

 so hardly won by generations of workers is being codified and 

 properly utilised in developing a scientific conspectus. 



In Radium a substance has been discovered which decom- 

 poses, apparently without rhyme or reason, at a perfectly 

 constant rate and in so doing gives out an amount of energy 

 altogether extraordinary in comparison with that given out in 

 any of the cases of chemical change known previously — 

 hundreds of times as much as can be derived from the com- 

 bustion of an equal weight of coal. And the process is a 

 very slow one in some of its stages, though very rapid in 

 others ; judging from the rate at which change is observed to 

 take place, about 2,500 years may be expected to elapse before 

 any given quantity is entirely dissipated. 



If it be desired to form a picture of what is going on, 

 we may imagine a vast heap of similar live shell — shell 

 charged with an explosive — and that, in a given interval of 

 time, a certain proportion of these explode spontaneously but 

 without affecting the remainder; moreover, that in each 

 subsequent similar interval of time always the same propor- 

 tion of the remainder explode : obviously a smaller number 

 will be destroyed at each successive explosion. Such is the 

 behaviour of Radium. But to make the analogy complete, 

 the shell must be thought of as packed with shot together 

 with smaller shell; when these smaller shell escape, they in 

 turn explode and disperse both shot and shell. But the rates 

 at which the various smaller shell break down are different 

 from that at which the parent Radium shell explode. And the 

 radium shell, it is supposed, are derived from still more 

 complex shell — from Uranium, which breaks down so gradually 

 that its complete conversion is estimated to occupy eight 

 thousand million years. 



When Radium was discovered, it was entered among the 

 chemical elements in the metallic class, because it behaved 

 like a metal in forming salts. When the further discovery 

 was made that its radioactivity was consequent on its resolu- 

 tion into other substances, a dream was fulfilled which 

 Mendeleeff had caused not a few chemists to dream by intro- 

 ducing the celebrated Periodic system of classification— a 

 system which meant, if it meant anything at all, that the 



