296 PHYSICAL SCIENCE 



has some bearing on the problem of Saturn's 

 rings. 



A curious conclusion may be drawn from 

 the theory of the radiation-force between small 

 bodies. Unless the temperatures are the same, 

 the force on one need not necessarily be equal 

 to the force on the other : action and reaction 

 it seems are not equal and opposite. The incon- 

 sistency is, of course, prevented if ,we remember 

 that the momentum of the radiation must also 

 be taken into account. In reality each body is 

 emitting a stream of momentum which exists 

 for a while in the medium. In the interaction 

 between radiation and either body, Newton's 

 laws may still hold. Constantly the energy and 

 momentum of radiation seem to be exchanged 

 with those of matter, and to be just as much 

 physical realities. 



If we neglect this last effect, there is no 

 reason, in the case considered, why action and 

 reaction should be equal and opposite. It is 

 even possible to imagine the gravitation-pull and 

 the radiation-push so adjusted that the accelera- 

 tions become equal but in the same direction. 

 The hotter body will then chase the colder body 

 through space with constantly increasing velocity. 

 A limit will, however, eventually be reached, for, 

 owing to the Doppler principle, the waves in 

 front of a moving body are crowded up, and 

 those behind it lengthened out. The radiation- 

 pressure in front is thus increased, and that 

 behind diminished, so that the net result 

 is a retardation which tends to check the 

 motion. In the case of meteorites small but 

 yet large enough for the gravitative pull to be 



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