Tlic Maintenance of Energy. 15 



a maintenance of energy as would satisfy geological 

 requirements, let us assume that the whole was at the 

 absolute zero of temperature. We regard this nebula then 

 as having at tirst a uniform density throughout, a uniform 

 composition throughout, and at the absolute zero of 

 temperature, i.e., with an entire absence of molecular motion. 

 Now its components cannot have been in a gaseous state, 

 since a gas, to exist as such, requires molecular motion, 

 neither can they liave been liquid. They must therefore 

 liave been in the solid state of matter. Let us now conceive 

 the whole volume condensing, or growing smaller, by 

 gravitation to the centre of gravity of the whole. It is 

 evident that the matter nearest to the centre of gravity 

 would eventually be the matter at the centre of the sun. 

 Now the heat generated by the collision of a ijilling body 

 varies as the square of the velocity of that body at the point 

 of impact. But the matter nearest to the centre of gravity 

 would have the least distance to fall, and the force of gravity 

 would there be least. Its velocity thei'efore at point of 

 impact would be least, so that less heat would be generated 

 by the falling together of any given quantity of matter near 

 to the centre, than would be generated by tlie falling together 

 of an equal quantity situated at a further distance from it. 

 A body therefore formed in this way would be relatively 

 cold at the centre, and the temperature would increase in a 

 definite ratio towaids the surface. The heat generated would 

 not wholly be lost l)y radiation into space ])art of it would, 

 by conduction, heat the cooler matter nearer to the centre 

 and would perform work. 



On this view of its formation we may then conclude, that 

 the sun was never wholly in a gaseous or plastic state, that 

 its centre at first was cool and solid, that its formation was 

 attended by the gradual development of heat, that as heat 

 was generated the more volatile substances became vapours, 

 that on approaching the surface the heat was much more 

 than sufficient to turn all substances into a gaseous state. As 

 the cooler matter in the interior became heated by 

 conduction, the various materials would on arriving at their 

 successive critical temperatures turn to vapour. In dealing 

 with the question of the maintenance of the sun's energy we 

 have to consider the action ot two forces, namely, the 

 contraction of the matter in a gaseous state on its surface, 

 and the expansion from excess of heat of the materials nearer 

 the centre. The question of the duration of the sun's energy 



