i88i.] 
Latent Heat . 
641 
degree of convention, but there is no good reason to believe 
that sensible heat is transferred to interspheral matter in 
this mode. And as radiated heat may not affedt the temper- 
ature of the matter of space, it is quite possible that the 
diversity in temperature is not being, and cannot be, rec- 
tified. 
But heat exchange between the spheres is aftive. Every 
touch of a solar ray on a planet conveys heat energy from 
the sun to the planet. Thus the inequality of sensible heat 
between the orbs of space is being slowly overcome. This 
inequality is not alone a result of the more rapid cooling of 
the smaller orbs, but arises largely from the differences in 
gravitative vigour of the orbs. Gravity on the sun is much 
greater than on the earth. Compression of the internal 
matter of the sun must greatly exceed that of the earth. 
Consequently the specific heat of solar substance must de- 
crease, and its latent heat become sensible, to a much greater 
extent than in the earth. In consequence of this the earth 
could never have reached the temperature of the sun, and 
possibly its highest temperature was far below the present 
solar temperature. In like manner the highest degree of 
lunar temperature must have been considerably less than 
that of the earth. This is, doubtless, one main cause of the 
heat inequality between the larger and smaller orbs, the 
greater rapidity of cooling in the latter being the other 
cause. And from this results the long process of equilibra- 
tion to which all existing planetary activity is due. 
Yet it must not be supposed that when equilibrium of 
temperature is finally attained the spheres will cease to emit 
heat. For radiation from the spheres is not to other spheres, 
but to the matter of space, and the falling of these rays on 
other spheres is, in a certain sense, accidental. When, 
however, temperature equilibrium is established between the 
spheres, thenceforward each will absorb as much radiant 
heat as it emits, and the radiations of heat into space will 
be just balanced by the receptions of heat from space. Yet 
this final equilibrium of heat emission will not be an equili- 
brium of temperature. If we take the sun and the earth as 
illustrations, we find the earth receiving heat from the whole 
hemisphere of the sun, and returning heat only from its own 
much smaller hemisphere. The area of the sun’s hemisphere 
is 12,000 times that of the earth, and if the temperatures of each 
were equal, and each radiating heat with equal energy to the 
other, the total heat received by the earth from the sun must 
considerably exceed that received by the sun from the earth. 
Necessarily, then, the surface temperature of the sun must 
