TEMPER ATURE OF SPACE. 37 



whole solar system, receives its internal heat from without 

 while passing through hot and cold regions.* 



The question whether the thermal conditions of" the celes- 

 tial regions, and the climates of individual portions of space, 

 have sufiered important variations in the course of ages, de 

 pends mainly on the solution of a problem warmly discussed 

 by Sir William Herschcl : whether the nebulous masses are 

 subjected to progressive processes of formation, while the cos- 

 mical vapor is being condensed aroimd one or more nuclei in 

 accordance with the laws of attraction ? By such a con- 

 densation of cosmical vapor, heat must be liberated, as in 

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

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

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

 may assume that all nebulae, including those which the high- 

 est power of optical instruments has hitherto failed in resolv- 

 ing, are closely crowded stellar swarms, our faith in this per- 

 petually augmenting liberation of heat must necessarily be 

 in some degree weakened. But even small consolidated cos- 

 mical bodies which appear on the field of the telescope as 

 distinguishable luminous points, may change their density 

 by combining in larger masses ; and many phenomena pre- 

 sented by our own planetary system lead to the conclusion 

 that planets have been soUdified from a state of vapor, and 

 that their internal heat owes its origin to the formative pro- 

 cess of conglomerated niatter. 



It may at first sight seem hazardous to term the fearfully 

 low temperature of the regions of space (which varies be- 

 tween the freezing point of mercury and that of spirits of 

 wine) even indirectly beneficial to the habitable climates of 

 the earth and to animal and vegetable life. But in proof of 

 the accuracy of the expression, we need only refer to the ac- 

 tion of the radiation of heat. The sun-warmed surface of 

 our planet, as well as the atmosphere to its outermost strata, 

 freely radiate heat into space. The loss of heat which they 

 experience arises from the difference of temperature between 

 the vault of heaven and the atmospheric strata, and from the 

 feebleness of the counter-radiation. How enormous would 

 be this loss of heat,$ if the regions of space, instead of the 



* See Poisson, Thione Mathim. de la Chaleur, p. 438. Accordinff 

 to him, llie consolidation of the earth's strata began from the center, ana 

 advanced gradually toward the surface; »J 193, p. 429. Compare also 

 Cosmos, vol. i., p. 176, 177. t Cosmos, vol. i., p. 83, 84, 144. 



X " Were there no atmosphere, a thermometer Ireely exposed (at sun- 



