PERPETUAL MOTION. 



a lifetime, that the same rate of boiling is still 

 being maintained by the original small salt- 

 spoonful of radium. It never gets cooled down 

 to the temperature of the liquid air. It is only 

 another way of putting this to say that the 

 radium appears to be an inexhaustible source 

 of heat. 



The matter would have been easier to under- 

 stand if some loss of substance could be de- 

 tected, showing that there has been combus- 

 tion or some other chemical or physical change 

 going on of which the constant output of heat 

 could have been a product. But after the 

 longest period during which radium has been 

 observed at work producing heat and other 

 manifestations, it has been impossible subse- 

 quently by the aid of the finest balance to detect 

 the slightest loss of weight or any other chemical 

 change. It looked like a miracle, a suspension 

 of the great law of the conservation of energy. 

 It looked as if perpetual motion had at last 

 materialised. 



It was not long, however, after the phe- 

 nomena of radio-activity had first astonished 

 the world, before Professors Rutherford and 

 Soddy, of the McGill University, Montreal, 

 discovered an explanation which is now 

 generally accepted, and which brings the 

 behaviour of radium within the law of the 

 conservation of energy. 



The explanation will be referred to again in 

 179 



