A HISTORY OF SCIENCE 



reason to suppose that it is not an element in the 

 ordinarily accepted sense of the term." 7 



The Source of Energy of Radio-Activity 



In 1903 Messrs. Curie and Laborde 8 made the re- 

 markable announcement that a crystal of radium is 

 persistently warmer than its surrounding medium; 

 in other words, that it is perpetually giving out heat 

 without apparently becoming cooler. At first blush 

 this seemed to contradict the great physical law of 

 the conservation of energy, but physicists were soon 

 agreed that a less revolutionary explanation of the phe- 

 nomenon is perfectly tenable. The giving off of heat 

 is indeed only an additional evidence of the dissipa- 

 tion of energy to which the radio-active atom is sub- 

 jected. And no one now believes that radio-activity 

 can persist indefinitely without actually exhausting the 

 substance of the atom. Even so, the evidence of so 

 great a capacity to give out energy is startling, and 

 has given rise to various theories (all as yet tentative) 

 in explanation. Thus J. Perrin 9 has suggested that 

 atoms may consist of parts not unlike a miniature 

 planetary system, and in the atoms of the radio- 

 elements the parts more distant from the centre are 

 continually escaping from the central attraction, 

 thus giving rise to the radiations. Monsieur and 

 Madame Curie have suggested that the energy may be 

 borrowed from the surrounding air in some way, the 

 energy lost by the atom being instantly regained. 

 Filipo Re, 10 in 1903, advanced the theory that the 

 various parts of the atom might at first have been free 

 particles constituting an extremely tenuous nebula. 



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