ASTRO-PHYSICS 295 



inversely as the square of the distance, and will 

 thus balance each other at all distances. Particles 

 intermediate in size will follow intermediate paths, 

 AF, AG, AH, etc., while the dust which suffers 

 a resultant repulsion will fly away outside the 

 path ADE. As the comet swings round the 

 sun, the tail becomes expanded into the fan-like 

 form commonly observed. The head of the 

 comet goes on its way into the depths of space, 

 having lost some of the smaller constituents of 

 its tail, which are scattered throughout inter- 

 planetary regions. 



Not only does the radiation from the sun 

 cause a repulsion of small objects, but their 

 radiation to each other will, as Professor Poynting 

 has shown from the theory, lead to a mutual 

 repulsion when the bodies are placed in a region 

 of space where the effective temperature is lower 

 than their own. Two meteorites at ordinary 

 temperatures, say at 300^ on the absolute scale, 

 will in cold space repel each other with a force 

 equal to their mutual gravitative attraction when 

 their radii are about 3.4 centimetres, and, in the 

 case of smaller bodies, the repulsion will overcome 

 the gravitative effect. In this case, when the 

 gravitational force is that between bodies of 

 small mass, instead of that between some small 

 body and the gigantic sun, a resultant repulsion 

 is reached at much larger dimensions than those 

 of the case formerly considered. It is evident 

 that a swarm of meteorites of the riofht size 

 might continue to revolve round a planet or 

 sun without mutual forces and independently 

 of each other. It is possible that this result 



