CLASSICAL AND NEW MECHANICS 173 



its energy remains constant. If it gives out heat, 

 light, or electro-magnetic energy, or if it even moves, 

 its mass continually decreases, until, theoretically at 

 least, it would melt away into a complex of energy : I 

 know of no name to designate so immaterial a thing 

 as this complex of energy located somewhere in vacu- 

 ous space. To be sure, this interchange between mass 

 and energy is very slow, so slow that ordinary minds 

 grow skeptical of its existence. Thus a body radiat- 

 ing enough heat to warm a kilogram of water, one 

 degree Centigrade, would decrease in mass only about 

 4.6 X io~ n grammes. It is fortunate that mass dis- 

 sipates so slowly or we should indeed be things dreams 

 are made of. 



The last deduction I shall note is that the tem- 

 perature of a moving body is less when measured by 

 a moving observer than by one who is stationary. 



Such are some of the more startling conclusions 

 which have been derived from the Principle of Rela- 

 tivity. If they can be established, and men be per- 

 suaded that the universe is, or even may be, as they 

 indicate, a revolution has been accomplished in scien- 

 tific thought. If we are to criticise the conclusions of 

 this hypothesis, and they are opposed to what seems 

 reasonable, it must be done by examining the postu- 

 lates; in them, as in all hypotheses, lies the real 

 strength or weakness of the system. 



Professor Einstein asks us to accept two postulates. 



