MECHANICAL PRESSURE OF LIGHT; ITS CONSEQUENCES. 235 



tion, as the head approaches the sun evidence of iron, 

 magnesium and other metals becomes apparent. We can 

 easily see that as the comet approaches the sun the hydro- 

 carbons must break up into hydrogen gas and other hydro- 

 carbons of higher boiling point. Finally as the sun is ap- 

 proached and the heat becomes intense these hydrocarbons 

 themselves will break up into free carbon in the form of 

 soot. The whole process is analogous to that which takes 

 place in an ordinary gas flame with the exception that 

 since there is no air, the soot formed cannot burn into gas 

 but must exist in the form of small particles. These small 

 particles whether in the form of a mist of liquid hydro- 

 carbons, or in the form of a carbon soot, must fall under 

 the sway of the mechanical pressue of light. If they are 

 so small that the light-pressure overbalances the forces of 

 the sun's gravitative pull they will be driven back from the 

 comet with a speed depending on their size and will con- 

 stitute the ordinary comet's tail. If these small particles 

 vary in size, as would naturally be the case, the rate at 

 which the light drives them will vary and the resulting 

 tail will be curved. 



Since the material composing the comet is hetero- 

 geneous, one constituent after another will decompose and 

 several tails will be formed successively with curvatures 

 depending on the size of the particles. If the particles are 

 larger than can be repelled by the sunlight they will form a 

 tail pointing towards the sun, which is a rare phenomenon, but 

 occasionally observed. The sizes of the mist particles of the 

 comets' tails necessary to account for their obs3rved length 

 and curvature have been calculated, and are quite in the 

 order of particles we are familiar with on earth in the early 

 stages of combustion. They vary in diameter from 1 /10,000 

 to 6/1,000 of a millimetre. Now a particle one-half the 



