CELESTIAL EVOLUTION. 229 



esis by the famous German physicist, Hermann 

 Ludwig Ferdinand von Helmholtz (1821-1894), 

 in 1854, in his theory of the maintenance of the 

 Sun's heat. Many theories had been already 

 advanced to account for this. After the dis- 

 covery of the conservation of energy, Julius 

 Robert Mayer, one of the discoverers, put 

 forward the theory that the Solar heat was 

 sustained by the inflow of meteorites from space, 

 and this idea was developed in 1854 by Sir 

 William Thomson, now Lord Kelvin (born 

 1824), but it was soon apparent that the 

 supply of meteors required to sustain the Solar 

 heat was such as would have increased the 

 mass of the Sun very considerably. Accord- 

 ingly the hypothesis was partially abandoned, 

 and was succeeded by that of Helmholtz, who 

 pointed out that the radiation of the Sun's 

 heat was the result of its contraction through 

 cooling. The rate was then estimated at 380 

 feet yearly, or a second of arc in 6000 years. 

 This theory was at once generally accepted. It 

 assumes the Sun to be still contracting, and 

 therefore, on going backwards in imagination, 

 we reach a period when the Sun must have 

 been much larger than now, and, in fact, 

 extended beyond the orbit of Neptune. 



Several objections to Laplace's nebular theory 



