196 PROF. O. REYNOLDS ON COMETARY PHENOMENA. 



the consequent attraction it has for its atmosphere ; for it 

 is clear that there must be a limit to the pressure which 

 the atmosphere can exert on the surface of the body ; and 

 this limit must depend on the size of the body. Up to a 

 certain point the thicker the atmosphere the greater would 

 be the pressure on the surface ; yet there must be a limit 

 beyond which the extension of the atmosphere would pro- 

 duce no effect, a limit beyond which the external air would 

 be so distant that the central body would not exert suffi- 

 cient coercive force to retain it, so that all excess would go 

 off, expanding into space. Hence the temperature at which 

 evaporation will continue on any body must depend on the 

 size of the body. The smaller the body, the lower will be 

 this temperature. 



If, then, the body is so small that the atmosphere, when 

 at its greatest, cannot restrain evaporation until the tem- 

 perature is equal to the temperature of equilibrium, then 

 the body will go on evaporating and the vapour go on 

 expanding into space until it is all evaporated, or, at any 

 rate, until all the softer materials, those which evaporate 

 at a low temperature, are gone. Thus we see that no body 

 whose temperature of equilibrium remains fixed — that is to 

 say, no body which moves round the sun in a circle — no 

 planet, in fact, can remain for ever in a condition of perma- 

 nent evaporation ; for in time, no matter how long, it would 

 lose all those materials on its surface which would evaporate 

 at a temperature below the temperature of equilibrium. 

 This, then, is the reason why there is not permanent 

 evaporation going on on the earth. The temperature of 

 equilibrium may be taken roughly at something like 50 , 

 whereas the temperature at which the most volatile 

 material (water) will boil is 21 2°. If, however, the earth 

 were to approach the sun until its temperature of equili- 

 brium rose to 300 , then the water would commence 

 evaporating until either the pressure of the atmosphere 



