ADVANCE OF ASTRONOMY. 209 



the direct result of the condensation. Most of 

 this has already been lost ; but as the cooling proceeds, 

 further condensation of the interior (gases) ensues, 

 and this implies further evolution of heat. Thus 

 as the sun parts with heat it compensates for its 

 loss by evolving more. In brief, gravitational energy 

 is exchanged for radiant energy. Ho\v long it can 

 continue to do so before ceasing to glow, before fad- 

 ing away into a dark star, is really indeterminable 

 in the present state of our knowledge of the sun's 

 physical constitution, but some rough calculations 

 have been made. Helmholtz estimated the rate of 

 the sun's contraction at about 220 feet a year, and 

 granted a lease of life for many millions of years to 

 come. 



Whether the sun is at present becoming actually 

 cooler we do not certainly know, but it is interesting 

 to take note of Lane's theorem (1870), which, on the 

 assumption that the sun is gaseous and behaves as 

 a perfect gas (one whose relations of volume to pres- 

 sure are indicated by Boyle's Law), seeks to show 

 that the temperature must be increasing, not decreas- 

 ing. As we cannot assert that the behaviour of gases 

 in the sun's interior is such as Boyle's Law indicates, 

 we cannot at present decide whether the sun has yet 

 attained its maximum splendour or whether it has 

 now begun to wane. 



Collisions and Impacts. From what has been 

 said it is evident that the picture of the sun's origin 

 which astronomers incline to give, is that of a vast 

 primitive nebula, with a great store of energy in the 

 mutual gravitation of its parts. We have also 

 noted the importance of the suggestion due to Helm- 

 holtz that cooling induced shrinkage, and that this 

 in turn evolved more heat But another possible 



