PHYSICAL PARADOXES. 



2. The Inexhaustible Source of Heat. 



When a jug of hot water is placed on the 

 table, we know that before long it grows cool, 

 and in time cold. A foot-warmer keeps con- 

 stantly losing heat until it needs changing. 

 Universally, according to our experience, a body 

 which is hotter than its surroundings gives up 

 heat to them until it is of the average tem- 

 perature. 



The ice in our experiment of the last section 

 but one, after boiling the liquid air for some time, 

 sinks at last to the temperature of the liquid 

 itself, to which it has given up all its excess 

 of heat, and thereafter it can do no more 

 boiling. 



But there has been discovered lately what 

 appears to be an exception to this general rule 

 of the dissipation and equalisation of heat- 

 energy. 



If radium is put into liquid air, as it was by 

 the late Professor Curie, it behaves at first like 

 the ice, causing brisk vaporisation, and boiling, 

 which presently dies down. But with radium 

 it never ceases altogether. A little boiling of 

 the liquid goes on constantly. If the liquid 

 air be put away for twenty-four hours, with the 

 radium in it, the latter will still be slowly 

 boiling it. If the level of the liquid air be 

 maintained by fresh supplies, it will be found 

 at the end of a month, of a year, apparently of 



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