40 Trans. Acad. Sci. of St. Louis. 



perimentally determined by Wilson and Gray (Phil. Trans., 

 1894), which is about 8000° C, we see that when the Sun's 

 radius was twice as great as at present, the effective tempera- 

 ture, by the above law, was about 4000° C. ; and when the 

 radius had eight times its present value, the temperature was 

 only 1000° C, which would not fuse the more refractory metals. 

 The following table shows the effective temperature of the 

 solar nebula when it extended to the several planets : — 



CAbsolute 

 Temperature.) 



Present solar surface 8000° C 



Mercury 92° C. 



Venus 53° C. 



Earth 40° C. 



Mars 24° C. 



Jupiter 7° C. 



Saturn 4° C. 



Uranus 2° C. 



Neptune 1° C. 



The excessively low temperature of the solar surface when 

 it reached the orbits of the several planets can hardly fail to 

 excite our astonishment. The temperature was always much 

 below zero, and the density of the mass necessarily very small. 

 About the only escape from such low temperatures for the 

 planets at their formation is to suppose that the Sun has long 

 passed its maximum temperature, and as now cooled down 

 does not allow us to trace the past history of its temperature ; 

 but of course such an hypothesis is embarrassed by many 

 difficulties. Indeed it seems positively contradicted by the 

 existence of life upon our globe which could hardly have de- 

 deloped as Geology shows it did develop, had the Sun ever 

 been enormously hotter than at present.* The conclusion 

 that the planets were formed at very low temperatures there- 

 fore seems irresistible. 



* Some of these conclusions have been anticipated by Ritter, who re- 

 marks how contrary they are to current theories (herschende ansichten), 

 yet it does not appear that he made any very serious effort to overthrow 

 th? errors which have been handed down bv tradition. 



