400 Prof. De Volson Wood on 



atmosphere as a diathermanous medium, capable of absorbing 

 in different degrees the radiant heat from the sun and the 

 dark heat from the earth, deduced for the heat of space — or, 

 as he and Fourier called it, the stellar heat — approximately 

 -142° C.* (-287° F.), which is about 174° F. above absolute 

 zero. It is well known that Pouillet's data were imperfect, 

 several important elements being neglected, notably that of 

 the humidity of the air ; still it is not only the first, but, so 

 far as we know, the only attempt to formulate this relation. 

 It served to show, what has since been indicated by more 

 direct experiments, that the temperature of space is very low. 

 The delicate experiments of Professor Langley, before referred 

 to, show a great difference in the degree of absorption by our 

 atmosphere of different wave-lengths. The mean of the 

 values for nine different wave-lengths, treated by M. Pouillet's 

 formula, gives 139° F. above absolute zero, and the smallest 

 value of absorption, which was for the infra-red, gives only 

 71° F. above absolute zero for the heat of space. 



The heat of space may be considered as composed of three 

 parts: — (1) stellar heat, (2) the heat contained in the dark 

 matter of space, (3) the essential heat of the aether. 



1. By the stellar heat we mean the heat received directly 

 from the stars. It is a matter of easy calculation that, if the 

 50,000,000 of stars supposed to be visible with the most 

 powerful telescopes were all at the distance of the nearest 

 fixed star (a Centauri), or 221,000 astronomical units from 

 the earth, and if each radiated the same amount of heat as our 

 sun, the intensity varying as the inverse squares of the distances, 

 the earth would receive from them all less than — ^- as much 

 heat as it now receives from the sun. And when we consider 

 that only a very few stars are within measurable distances, 

 and that the remote ones may be, when compared with these, 

 well-nigh infinitely distant, it is evident that the amount of 

 heat received from the stars is insignificant and may be 

 discarded at the earth. 



2. It is certain that there is a large amount of dark matter in 

 space, since the meteoric dust and meteorites must come from 



* Comptes Rendus, 1838, vol. vii. p. 61. Pouillet's formula is 



<*''= 1-236 f^,- 0-489, 

 2 — 6 



in which 6'=the absorptive power by the atmosphere of the sun's heat, 



6= the absorptive power of terrestrial heat, 



t'=the temperature of the stellar heat, 



« = 1-0077. 

 If 6 = 1, its maximum, 6' = 02, we find *'=-235 c C. (-391° F.), or 

 71° F. above absolute zero. 



