182 comets' tails, the corona, and the aurora. 



vapors on the sido toward the sun. The hydroo-en t)oils off and the 

 vapors condense into small drops of hydrocarbons with higher l)oiling' 

 points, or idtiuiately solid carbon is thrown out, linelv divided as in 

 an ordinary Haine. The largest of these particles fall back to the 

 comet, or. if they are not condensed till at a great distance from it, 

 they form tails turned toward the sun. The smaller are driven rap- 

 idly from the sun l)y the pressure of its light, with a speed depending 

 on their size, and form the ordinary tails pointing" away from it. 

 That particles of different sizes should ])e formed from the same 

 comet is natural, since the comet is likeh- to be formed of heteroge- 

 neous matei-ials and there must be great variety in the circumstances 

 of condensation. Thus the comet of 1744 had no less than live tails of 

 different curvature. Occasionally the calculated repulsion on the 

 same tail is not found to follow exactly the law of the inverse square 

 of the distance from the sun throughout its whole length. This puz- 

 zling circumstance is at once explained if the particles should for any 

 reason change their state of aggregation, and coiseciuently their size, 

 during their headlong career. In the light of this theoiy the follow- 

 ing passages will l)e found ^'ery suggestive. 

 Herschel, Outlines of Astronomy (p. 376): 



""It is for the most part after thus passing the sun that they shine 

 forth in all their splendor, and that their tails acquire their greatest 

 length and development, thus indicating plainly the action of the sun's 

 rays as the exciting cause of that extraordinary emanation." 



Again (p. 560): 



"The tail of the great comet of 1680 immediately after its perihe- 

 lion passage was found by Newton to have been no less than 20,000,000 

 leagues in length and to have occupied only two days in its emission 

 from the comet's bod}^ — a decisive proof this of its Ijeing darted forth 

 by some active force, the origin of which, to judge by the direction of 

 the tail, nuist be sought in the sun itself.'' 



Now, a particle with one-half the critical diametei* would in the 

 course of traveling from the sun's surface to a distance equal to his 

 radius acquire a speed of 430 kilometers per second. With this 

 velocity it would cross a space equal to the diameter of the sun, 

 865,000 miles, in less than an hour. In comets' tails we probably 

 have to do with particles having less than one-eighteenth of the crit- 

 ical diameter. Such particles would cover the same distance in less 

 than four minutes. With a force majiy times the sun's attraction 

 driving them into space, they would make little of 20,000,000 leagues 

 in two da3's; whereas if this were to be accomplished against gravit}' 

 the velocity of projection required might well stagger the astronomers. 



Referring to Ilalley's comet, Hersclud says (p. 381): 



"On the 2d of October (the very day of the first observed com- 

 mencement of the tail) the nucleus, which had been faint and small, 



