No. 2.] BRITISH ASSOCIATION MEETING. 137 



round the sun. (Frankland and Lockyer find the yellow promi- 

 nences to give a very decided bright line not far from D, but 

 hitherto not identified with any terrestrial flame. It seems to 

 indicate a new substance, which they propose to call Helium.) 

 I believe I may say, on the present occasion, when preparation 

 must again be made to utilize a total eclipse of the sun, that the 

 British Association confidently trusts to our Government exercis- 

 ing the same wise liberality as heretofore in the interests of 

 science. 



The old nebular hypothesis supposes the solar system and other 

 similar systems through the universe which we see at a distance 

 as stars, to have originated in the condensation of fiery nebulous 

 matter. This hypothesis was invented before the discovery of 

 thermo-dynamics, or the nebulas would not have been supposed 

 to be fiery ; and the idea seems never to have occurred to any of 

 its inventors or early supporters that the matter, the condensation 

 of which they supposed to constitute the Sun and stars, could 

 have been other than fiery in the beginning. Mayer first sug- 

 gested 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 tempera- 

 ture of the Sun is due. Further, he made the important obser- 

 vations 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 2:enerated, and that thus 

 we can conceive the Sun even now to possess a sufficient store of 

 energy to produce heat and light, almost 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 gravitational explanation of future Sun-heat 

 amounts really to showing that the Sun's thermal capacity is 

 enormously greater, in virtue of the mutual gravitation between 

 the parts of so enormous a mass, than the sum of the thermal 

 capacities of separate and smaller bodies of the same material and 

 same total mass. Reasons for adopting this theory, and the con- 

 sequences which follow from it, are discussed in an article ' On 

 the Age of the Sun's Heat,' published in Mdcm'dUnis Magazine 

 for March, 18G2. 



