RADIATION IN TH K SoLAK SYSTEM. 



191 



cio-lit into cio-ht moi-c ('(|ii:il ololtcs. Xiz-aIw (lie nu I in (ion pressure 

 would 1h' iloiihlcd. while ii,r:i \ i(;it ioii would Itc llic siiiiic. 



Contiiiiio the process, aiul it is evident that by siuressive division 

 we, slioidd at last an-ive at o-lobes so small and witli total sui-faces 

 so t>-i-ea( that the pressure of the radiation would halaiice the pidl 

 of oravitatioii. Mere arithmetic shows that tliis balance^ would 

 occur when the earth was divided uj) into little spheres each one forty- 

 thousandth of a centimeter in diameter. 



In other Avords, a little si)eck one foi'ty-thousandth of a centi- 

 meter, say one one-hundred-thousandth of an inch in diameter-, and of 

 density ('(jual to that of the earth, would be neither attracted nor 

 repelled by the sun. 



This balance would hold at all distances, since both would vary 

 in the same way with the distance. Oui- arithmetic comes to this, 

 that if the earth wei-e spread out in a thin si)herical shell with radius 

 about four times the distance of Neptune, the i-epulsion of sunlight 

 falling on it would balance the inward pull by the sun and it would 

 have uo tendency to contract. 



With further division repulsion would exceed attraction, and the 

 i:»articles would be driven away. But I nnist here say that the law 

 of repulsion does not hold down to such line divisi(m. The repulsion 

 is somewhat less than we have calculated, owing to the diU'raction of 

 the light. 



Some very suggestive speculations with regard to comets' tails 

 have arisen from these considerations, and to these Professor Boys 

 directed the attention of section A last year. We may imagine that 

 the nucleus of a comet consists of small meteorite^. AAlien these 

 come near the sun they are heated and explosions occur, and fine 

 dust is produced not previously present. If the dust is sulHciently 

 fine, radiation may overpower gravitation and drive it away fi-om the 

 sun, and Ave may have a manifestation of this expelled dust in the 

 tail of the comet. 



I do not, however, want to dwell on this to-day, but to look at the 

 subject in another way. 



Let us again introduce our small black sphere, and let us make 

 it 1 cm.- in cross section, l.i;') cm. in diameter, and of the density 

 of the earth. The gravitation pull on it is forty-two thousand times 

 the radiation pressure. 



Now let us see the effect of size on the radiating body. Let us 

 halve the diameter of the sun. lie would then have one-eighth 

 the mass and one-cpiarter the surface. Or. while his jndl was re- 

 duced to one-eighth, his radiation push would only be reduced to 

 one-quarter. The pull would now be only twenty-one thousand 

 times the push. Halve the diameter again, and the pull would be 

 only ten thousand five hundred times the push. Reduce l!ie diam- 



