RADIATION IN THE SOLAR SYSTEM. 191 



eight into eight more equal g-lohes. Again the i-adiation pi'essiire 

 wonkl be (l()nl)le(l, while gravitation would he the same. 



Contiiuie the process, ^nd it is evident that by successive divisiou 

 Ave should at last arrive at globes so snuill aud with total surfaces 

 so great that the pressure of the radiation would balance the ])ull 

 of gravitation. Mere arithmetic shows that this balance would 

 occur when the earth was divided up into little si)h('i'es each one forty- 

 thousandth of a centimeter in diameter. 



In other words, a little speck one forty-thousandth of a centi- 

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

 density equal to that of the earth, would be neither attracted nor 

 repelled by the sun. 



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

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

 that if the earth weiv s])read out in a thin sj^herical shell with radius 

 al)out four times the distance of Nejitune, the repulsion of sunlight 

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

 have no tendency to contract. 



With further division re])ulsion would exceed attraction, and the 

 particles would be driven away. But T must here say that the law 

 of repulsion does not hold down to such fine division. The repulsion 

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

 the light. 



Some very suggestive speculations with regard to comets' tails 

 have arisen from these considerati(ms, 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^, ^^"h('n these 

 couie near the sun they are heated and ex])losions occur, and fine 

 dust is produced not j)reviously present. If the dust is sulHciently 

 fine, radiation may over])ower gravitation and drive it awa\' fi'om the 

 sun, and we ma}^ have a manifestation of this expelled dust in tlu^ 

 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.b") cm. in diameter, atid of the density 

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

 tlie radiation pressure. 



Now let us see the etl'ect of size on the radiating body. \jv{ us 

 halve the diameter of the sun. lie would then ha\-e one-eighth 

 the mass and (me-cjuarter the surface. Or, while his pull was re- 

 duced to one-eighth, his radiation push would only be i-educed 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. Keduce t!ie diani- 



