294 . PHYSICAL SCIENCE 



reflecting surface, the pressure to be detected 

 amounts to less than a milligram per square 

 metre. For an absorbing surface such as lamp- 

 black, the pressure is half as great as for a 

 reflector, and it is the difference between these 

 two effects that M. Lebedef has detected, the 

 results of unequal heating and of molecular 

 recoil being successfully eliminated. By another 

 method the same pressure was also demonstrated 

 by Nichols and Hull. 



Owing to this pressure, two bodies radiating 

 towards each other will experience a mutual 

 repulsion, which, for small particles, may over- 

 come the gravitational attraction. Even the 

 attraction of the sun on a body may be 

 neutralised if the body is of minute size, for the 

 radiation effect depends on the area of surface, 

 while the weight depends on the volume. As 

 the size is diminished, the area decreases less 

 rapidly than the volume, and, for microscopic 

 particles less than o.oooi millimetre in diameter, 

 the radiative repulsion of the sun becomes greater 

 than the gravitational attraction. An interesting 

 application of this principle has explained the 

 curious phenomena of comets' tails, which have 

 long puzzled the ingenuity of astronomers. If, 

 as is probable, a comet consists of a collection 

 of meteorites, varying in size from small worlds 

 to microscopic particles, on approaching the sun 

 the large masses will follow the parabolic path 

 ABC (Fig. 40), indicated by the ordinary 

 gravitational theory. Particles of the particular 

 size at which the radiative force just balances 

 that due to gravity will pursue a path, ADE, 

 in an undeviated course, for both the forces vary 



