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is much less than 212° F., which is the boiling point of water, 

 its most volatile substance; and we may assume that the 

 same is the case with all the other planets. If, however 

 the earth's atmosphere were removed, then evaporation 

 would go on until there was another atmosphere formed 

 which would hold the liquid in check. If, however, the 

 earth had no attraction for vapour, or only a very slio-ht 

 one, then it would go on evaporating, in the first place, until 

 all the water was ice, and then it would spend all the heat 

 it got from the sun in vapour. This, according to Sir J. 

 Herschel's rate, is sufficient to melt ice just enough to 

 reduce the diameter of the earth by an inch in about four 

 hours and a half, and if it had to evaporate the water as 

 well as melt the ice it would evaporate about one inch in 

 130 hours. Now, although this is a purely imaginary con- 

 dition with regard to the earth, yet it must exist in the 

 case of a small body like a comet; that is to say, there 

 would be no liquid on the comet even when evaporation 

 was going on, and, when the comet was near enough the 

 sun, permanent evaporation would go on, which would 

 only be ended by the comet removing itself, or by the 

 exhaustion of the volatile material. This latter would 

 take place supposing a comet should change its orbit when 

 near the sun into a circular orbit, like a planet or meteorite. 

 Even in the case of a periodic comet there must be some 

 exhaustion of the volatile materials. During the two hours 

 in which the comet of 1843 was within close approxima- 

 tion to the sun, if the comet had been made of ice covered 

 with lamp black it would have received the heat of 47,000 

 suns according to Sir J. Herschel's computation. Tliis 

 would have evaporated the ice at the rate of 55 feet per 

 hour on that side next the sun, or 13 feet over the whole 

 comet. But in fact, owing to the protection of its atmo- 

 sphere and imperfect absorbing power, it would have been 

 much less than this, that is to say, the diameter of the comet 



