36 
3. All the heat received by a body from the sun must be 
expended in one or other of the following ways : — 
I. By radiation from the body. 
II. By evaporating the materials. 
III. Producing chemical change in these materials, or in 
electrical separation, &c. 
That spent in the third method may be considered small. 
Thus 
the heat which a body receives=heat radiated + heat spent 
in evaporation (1) 
and 
heat radiated , . JS 
i — t • — t = (some constant) x 
liGcit received 
Now the temperature at which any given material, say 
water, would evaporate would be much lower on a comet 
than on a planet, on account of the comet being so much 
smaller. For we may assume that there is a limit to the 
pressure which an atmosphere of vapour of unlimited extent 
can exert on the materials of the body it envelopes, then 
the limit of the temperature of the body will be that 
which will evaporate the material of the body under this 
pressure. It is clear that if there be such a limit it must 
increase very rapidly with the mass of the solid body, and 
hence that it would be much higher in the case of a planet 
than in that of a comet. This temperature may be called 
that of permanent evaporation, for as long as it was main- 
tained the body would continue to evaporate; therefore the 
temperature of permanent evaporation of the planet would 
be much greater than that of the comet. That is, from 
equation (2,) the ratio of the heat radiated away to that 
received would be much less in the case of the comet than in 
the case of the planet, leaving, by equation (1), a greater 
ratio for evaporation in the former than in the latter. 
Now it is clear that our earth is well out of reach of this 
permanent evaporation ; for the temperature at the equator 
temperature of body . 
(distance of sun) a 
