PRESIDENTIAL ADDRESS, B.A. 1871. 185 



tute the Sun and stars, could have been other than 

 fiery in the beginning. Mayer first suggested that 

 the heat of the Sun may be due to gravitation : 

 but he supposed meteors falling in to keep always 

 generating the heat which is radiated year by year 

 from the Sun. Helmholtz, on the other hand, 

 adopting the nebular hypothesis, showed in 1854 

 that it was not necessary to suppose the nebulous 

 matter to have been originally fiery, but that 

 mutual gravitation between its parts may have 

 generated- the heat to which the present high 

 temperature of the Sun is due. Further, he made 

 the important observations that the potential 

 energy of gravitation in the Sun is even now far 

 from exhausted ; but that with further and further 

 shrinking more and more heat is to be generated, 

 and that thus we can conceive the Sun even now to 

 possess a sufficient store of energy to produce heat 

 and light, almost as at present, for several million 

 years of time future. It ought, however, to be 

 added that this condensation can only follow from 

 cooling, and therefore that Helmholtz's gravita- 

 tional explanation of future Sun-heat amounts 



