308 DISCOVERY CH. 



of life, for its activity is reduced by one-half in four 

 days. 



When first collected, radium emanation is more ener- 

 getic than radium itself, and will produce precisely the 

 same effects ; but it runs through its stock of energy in 

 a few weeks, whereas the activity of radium remains 

 practically unaltered for hundreds of years. Suppose a 

 ton of coal could be kept burning in a furnace continu- 

 ously for about two thousand years, then it would pro- 

 duce approximately the same energy as would be given 

 out by one gram (15 grains) of radium in the same 

 period. If the rate of atomic disruption could be 

 increased, and the life of radium therefore reduced, the 

 rate of output of energy would be proportionally greater. 



An illustration of the magnitude of the energy which 

 might thus be obtained has been given by Sir William 

 Ramsay. " Suppose," he said, " that the energy in a 

 ton of radium could be utilised in thirty years, instead 

 of being evolved at its invariable slow rate of 1760 years 

 for half-disintegration, it would suffice to propel a ship 

 of 15,000 tons, with engines of 15,000 horse-power, at the 

 rate of 15 knots for thirty years practically the life of 

 the ship. To do this actually requires a million and a 

 half tons of coal." 



When radium from which the emanation has been 

 collected by suitable means is left to itself for about a 

 month, it recovers completely its original condition, and 

 a fresh supply of emanation can be obtained from it. 

 In cases, therefore, where radium is required for any 

 purpose, but cannot be procured on account of its high 

 price, radium emanation provides a cheap substitute 

 possessing all the properties of its parent. Like the 

 widow's barrel of meal and cruse of oil, radium wasteth 

 not, neither does it fail, though a supply of emanation 



