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 , . .. temperature of body.^v 



- — : -. — Y =(some constant) x 7-rv-f ^ r^ — -(2) 



neat received ^ ^ (distance 01 sun)^ 



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 



