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 slight 
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. This 
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 
