44 COSMOS. 



physical probability to the hypothesis of Poisson, that the 

 different regions of space must have a very various tempera- 

 ture, owing to the unequal distribution of heat-radiating stars, 

 and that the earth, during its motion with the whole solar 

 system, receives its internal heat from without, while passing 

 through hot and cold regions. 29 



The question whether the thermal conditions of the celestial 

 regions, and the climates of individual portions of space, 

 have suffered important variations in the course of ages, de- 

 pends mainly on the solution of a problem warmly discussed 

 by Sir William Herschel : whether the nebulous masses are 

 subjected to progressive processes of formation, while the cos- 

 mical vapour is being condensed around one or more nuclei 

 in accordance with the laws of attraction? By such a 

 condensation of cosmical vapour, heat must be liberated, as 

 in every transition of gases and fluids into a state of solidifica- 

 tion. 30 If, in accordance with the most recent views, and 

 the important observations of Lord Rosse and Mr. Bond, we 

 may assume that all nebulae, including those which the highest 

 power of optical instruments has hitherto failed in resolving, 

 are closely crowded stellar swarms, our faith in this perpe- 

 tually augmenting liberation of heat must necessarily be in 

 some degree weakened. But even small consolidated cosmical 

 bodies which appear on the field of the telescope as distinguish- 

 able, luminous points, may change their density by combining 

 in larger masses ; and many phenomena presented by our own 

 planetary system lead to the conclusion, that planets have been 

 solidified from a state of vapour, and that their internal heat 

 owes its origin to the formative process of conglomerated matter. 



29 See Poisson, Theorie Mathem. de la Chaleur, p. 438. 

 According to him, the consolidation of the earth's strata 

 began from the centre, and advanced gradually towards the 

 surface; 193, p. 429. Compare also Cosmos, vol. i. p. 169. 



30 Cosmos, vol. i. pp. 67, 134. 



