PORTION OF THE COSMOS. COSMICAL SPACE. 39 



also alter their density in combining into larger masses ; and 

 many phenomena which our own planetary system presents 

 lead to the supposition that the planets have been condensed 

 from a state of vapour, and that their internal heat is owing 

 to this process. 



At first sight it seems hazardous to assert that a tempe- 

 rature of space so very low as between the freezing 

 points of mercury and of spirits of wine, can be deemed, even 

 indirectly, beneficial to the habitable climates of the globe, 

 and to the life of plants and animals ; but in order to be sa- 

 tisfied of the correctness of the expression it is sufficient to 

 reflect on the influence of the radiation of heat from the 

 earth. The surface of our globe warmed by the solar heat, 

 and the atmosphere up to its outermost stratum, radiate freely 

 towards space. The loss of heat which they suffer arises 

 from the difference of temperature between the air and space, 

 and the feebleness of the return which they receive. How 

 enormous would be the loss ( 77 ) if space, instead of the tem- 

 perature which we express by 60 Cent. (76 F.), had, 

 for example, a temperature of -800 Cent. (-1408 P.), 

 or even several thousand degrees lower ! 



There still remain to be developed two more considerations 

 in reference to the existence of a fluid throughout space : 

 one, less well-established, relates to a " limit to the trans- 

 parency of space ;" the other, based on direct observation, 

 and affording numerical results, to the regular diminution 

 of the period of revolution of Encke's comet. Olbers of 

 Bremen, and, as Struve has remarked, Loys de Cheseaux 

 at Geneva, eighty years before, ( 78 ) called attention to the 

 dilemma, that as in infinite space no point can be imagined 



