April 23, 1909] 



SCIENCE 



657 



between the mass of an electron and its velocity. 

 It is, moreover, strikingly similar to the equations 

 that have been obtained for electromagnetic mass. 



The new view leads to an unusual conception 

 of the nature of light. It offers theoretically a 

 method of distinguishing between absolute and 

 relative motion. 



Mass is defined by Professor Lewis as mo- 

 mentum {M) divided by velocity («), 



m =M/v. 



I should like to say a few words about tbis 

 summary and tbe paper to which it belongs. 



The notion of momentum in a beam of 

 radiation is introduced with the aid of the 

 "law of conservation of momentum." The 

 other two laws required, of the three in all, 

 are the conservation of energy and tbe con- 

 servation of mass. 



For the sake of argument, I shall assume a 

 beam of radiation to consist of a mass in 

 motion and proceed to consider the use of such 

 a hypothesis or conception. 



What happens when that beam impinges on 

 a body? That the body receives energy and 

 that this energy is shown by the movement 

 of the body is settled beyond doubt by experi- 

 ment, but that the moving mass in the beam 

 sticks to the body it strikes is very question- 

 able. How can it stick to a body which radi- 

 ates as much energy as it receives and of the 

 same nature? Professor Lewis does not seem 

 to consider this difficulty. But, for the sake 

 of argument again, I assume that what is mass 

 in the beam of radiation does adhere to the 

 body it strikes. Then, of course, the mass of 

 the body struck increases as it moves and 

 increases as it receives this particular form 

 of energy, but only as it receives this par- 

 ticular form. Yet Professor Lewis considers 

 this increase of mass with energy as typical 

 and concludes that because the mass of a body 

 increases as it receives radiant energy, to 

 which he assigns a very special constitution, 

 therefore its mass increases when it receives 

 any energy whatsoever and diminishes when it 

 loses any energy whatsoever. Otherwise, what 

 does the following mean : 



Assuming the fundamental conservation law 

 [of momentum? O. L. S.], we must regard mass 



as a real property of a body which depends upon 

 its state and not upon its history. Hence it is 

 obvious that if in any other way than by radia- 

 tion the body gains or loses energy, it must gain 

 or lose mass in just the above proportion [see 

 equation (5) below, C. L. S.]. In other words, 

 any change in a body's content of energy is accom- 

 panied by a definite change in its mass, regardless 

 of the nature of the process which the energy 

 change accompanies. 



This seems to me equivalent to saying that 

 all energy is of the same nature as radiant 

 energy, a notion not acceptable in the present 

 state of our sciences. Professor Lewis thinks 

 that consequently one of the axioms of New- 

 tonian mechanics must be changed. I suppose 

 he refers to axiom 1, but none of the three 

 says a word about this relation. They imply 

 this independence of mass and velocity, but 

 were they to be found dependent, I can not 

 see that any of the three would be changed, 

 necessarily, in wording. I do not find in this 

 whole development anything more than a spe- 

 cial kind of action, one that can not be 

 generalized at all. A ship bombarded by pro- 

 jectiles and moving in the same direction as 

 the projectiles continues in the same direction 

 as before with increased mass and increased 

 velocity due to the mass and energy of those 

 missiles. But who would draw any general 

 conclusions as to the nature of all the other 

 energies from this? It is a very easily an- 

 alyzed case, but I do not see how it differs in 

 principle from the more obscure one of radiant 

 energy. 



The change in mass for a given quantity of 

 energy is calculated by Professor Lewis thus: 



The moving mass of the beam imparts dE 

 of energy in dt time, so in t time it imparts 

 (^dE/dt)t of energy. During this time t, a 

 quantity of energy has traveled up to the 

 body absorbing the radiation and been de- 

 livered to it equal to fs where f is the radia- 

 tion pressure and s is the distance the radia- 

 tion has traveled in t time. Making t equal 

 to unity, s becomes the velocity of radiation, 

 V. Then, 



f = dE/Vdt. (1) 



By condition, this f, being due to a moving 



