PROF. O. REYNOLDS ON COMETARY PHENOMENA. 197 



of vapour was sufficient to stop further boiling, or else until 

 it was all gone*. Or if, on the other hand, the size of 

 the earth were reduced so that it was no longer able to 

 retain an atmosphere whose pressure on its surface was 

 sufficient to prevent water boiling at 6o° F., then the 

 water would go on boiling until it was all consumed. We 

 see, then, that the earth owes its stable condition to being 

 so far away from the sun, and to being of such size that it 

 can retain an atmosphere sufficient to prevent its softest 

 material from evaporating at a temperature below that of 

 equilibrium, and that in all bodies where this is not the 

 case evaporation will be going on. We may now see why 

 comets should generally be in a state of evaporation, even 

 though they may not contain softer materials than water, 

 and may not approach nearer the sun than the distance of 

 the earth. The fact of their being so much smaller 

 prevents their retaining the same pressure of atmosphere ; 

 and so their materials evaporate at a lower temperature. 

 The eccentricity of their orbits is also essential to the 

 explanation ; for if they remained at a constant distance 

 they must eventually lose all the material which would 

 evaporate at that distance, and so become like the other 

 planets. This will be the case with periodic comets, those 

 which, in spite of the eccentricity of their orbits, return 

 again and again ; for each time they come near enough 

 to the sun for the temperature of equilibrium to rise 

 above that of evaporation they will lose some of their 

 softer materials, until these are all done ; and then, so 

 far as evaporation is concerned, the comets will behave 

 as planets. 



This may appear as though it were incompatible with 

 the existence of periodic comets. It is not so, however; 

 it is only incompatible with the permanence of periodic 



* It is true that water evaporates from its surface at a temperature much 

 below 2 1 2 n ; but an atmosphere of steam would soon prevent this. 



