SOME PHYSICAL PROBLEMS 



troduced, whereupon the mercury in the tube gradually 

 rises, falling again when the radium is removed. By 

 careful tests it has been determined that a gram of 

 radium emits about twenty-four hundred gram-calories 

 in twenty-four hours. On this basis a gram of radium 

 in a year emits enough energy to dissociate about two 

 hundred and twenty-five grams of water. 



What seems most remarkable about this constant 

 emission of heat by the radium atom is that it does 

 not apparently draw upon external sources for it, 

 but maintains it by the internal energy of the atom 

 itself. This latent energy must be enormous, but is 

 only manifested when the atom is breaking up. In 

 this process of disruption many of the particles are 

 thrown off; but the greater part seem to be stopped 

 in their flight in the radium itself, so that their energy 

 of motion is manifested in the form of heat. Thus, 

 if this explanation is correct, the temperature of the 

 radium is maintained above that of surrounding sub- 

 stances by the bombardment of its own particles. 

 Since the earth and the atmosphere contain appreci- 

 able quantities of radio-active matter, this must play 

 a very important part in determining the temperature 

 of the globe so important a part, indeed, that all for- 

 mer estimates as to the probable length of time during 

 which the earth and sun will continue to radiate heat 

 are invalidated. Such estimates, for example, as that 

 of Lord Kelvin as to the probable heat-giving life of 

 the sun must now be multiplied from fifty to five 

 hundred times. 



In like manner the length of time that the earth has 

 been sufficiently cool to support animal and vegetable 



in 



