168 FROM NEBULA TO NEBULA 



solar gravity, but also to carry the particles outward 

 with the velocity of light itself. Expanding this idea, 

 they go on and picture to a wondering laity how come- 

 tary tails are shorn away in the very act of growing on, 

 and, waxing still more eloquent, how they are carried 

 away on the caudaliferous ether to distant bournes 

 where, foregathering with other such ghostly moultings, 

 they hatch out new planetary systems. Several mathe- 

 maticians have gained undying fame by calculating this 

 and that about the size of these motes and denning the 

 relative intensity of light and electric pressures as con- 

 trasted with gravity, etc., etc., so that the theory may 

 now be said to have as respectable standing among New- 

 tonians as the old standbys of uncaused motions, irredu- 

 cible momenta, and the rest. 



On the occasion of its initial visit to the sun the 

 comet is naturally charged with a greater cargo of gases 

 than at any subsequent apparition ; hence, after a few re- 

 turns, it ceases to be visible to the naked eye and then be- 

 comes telescopic only, or is lost altogether to human sight. 

 As long as the comet continues rich in occluded gases 

 (whether the visit be its first or not) and its perihelion 

 distance is sufficiently short, its advent is accompanied 

 by an interesting display of celestial pyrotechnics. After 

 playing its harlequin tricks for a gay month or two and 

 casting its fiery confetti in the eyes of the staid old 

 planets, it returns on its long road through the dark to 

 its home in aphelion. As it so retires, it collects its scat- 

 tered elements its integral attraction more and more 

 reasserting itself as the sun is left behind until, after 

 no long while, it rolls itself up into a ball as self-con- 

 tainedly as the earth herself. With each return, the ag- 

 ing comet becomes less flamboyant, its orbital eccentric- 

 ity less pronounced, its inclination less arbitrary; in fine, 

 it grows domesticated and turns into an asteroid. It is 

 not true, however, that all asteroids have once been 

 comets or, conversely, that all comets become in time 

 asteroids; for many of the asteroids are native to our 

 system, the same as the planets, while the comets may 

 graduate into satellites, or, indeed, may end their careers 



