the Formation of Comets' 1 Tails. 461 



just made has been made on the supposition that the tempera- 

 ture of the escaping gas is 0° C. If the absolute temperature 

 in Centigrade units of the gas be t, we must multiply the 



above estimate by a/ - — _. Now in no case has the rapidity 



of tail-formation greatly exceeded one million miles in three 

 days, except in the case of those comets which have approached 

 very near to the sun, and where, consequently, the tempera- 

 ture at which the evaporation has taken place must have been 

 very great. Donati's comet is one of the most striking ex- 

 amples of comets with large and rapidly formed tails which 

 have not approached very near to the sun; and in Donati's 

 comet the tail increased in length from 14 million miles on 

 August 30 to 51 million miles on October 10, or at an average 

 rate of somewhat less than a million miles in a day. 



Those comets which have formed large tails with exceptional 

 rapidity have approached very near to the sun. Thus the 

 comet of 1680, which formed a tail 60 million miles long in 

 two days during its perihelion passage approached so near to 

 the sun as to be exposed to a solar radiation 25,600 times more 

 intense than that to which the earth is exposed. To use Sir 

 John HerscheFs words, " In such a heat there is no solid sub- 

 stance we know of which would not run like water, boil, and 

 be converted into vapour or smoke." It seems probable that it 

 is only the shortness of the time during which the comet is ex- 

 posed to such a temperature which prevents its being altogether 

 converted into vapour. The smaller fragments of the comet 

 will, I conceive, be entirely evaporated ; and the last portions 

 of vapour from any fragment will, as I have shown, be carried 

 backwards with immense velocity into the tail. After this 

 vapour has arrived in colder regions, it seems probable that it 

 will condense and become visible as a cloud of finely divided 

 solid or liquid matter. In these cases then the visible tail will 

 consist, not of matter which has resisted evaporation, but 

 largely and perhaps almost entirely of matter which has eva- 

 porated and has recondensed. 



In conclusion, if this be the true explanation of the pheno- 

 menon of comets' tails, then every meteoric cloud of matter 

 which approaches sufficiently near to the sun to undergo rapid 

 evaporation must become tailed like a comet, as it passes 

 through its perihelion passage. I would suggest that we 

 may have here an explanation of the radiated structure of the 

 sun's corona. As different masses of meteoric matter approach 

 close to the sun, the smaller fragments will be almost or entirely 

 evaporated, a large portion of the vapour from them being 

 carried rapidly away from the sun, thus giving rise to a 



