->ti The Outline of Science 



heat. Combustion is a chemical reaction between atoms. The 

 conditions tliat make it possible are known and the results are 

 predictable and measurable. But no chemical reaction of the 

 nature of combustion as we know it will explain the sun's energy, 

 nor indeed will any ordinary chemical reaction of any kind. If 

 the sun were composed of combustible material throughout and 

 the conditions of combustion as we understand them were always 

 present, the sun would burn itself out in some thousands of years, 

 with marked changes in its heat and light production as the pro- 

 cess advanced. There is no evidence of such changes. There is, 

 instead, strong evidence that the sun has been emitting light and 

 heat in prodigious quantities, not for thousands, but for millions 

 of years. Every addition to our knowledge that throws light on 

 the sun's age seems to make for increase rather than de- 

 crease of its years. This makes the wonder of its energy 

 greater. 



And we cannot avoid the issue of the source of the energy 

 by saying merely that the sun is gradually radiating away an 

 energy that originated in some unknown manner, away back at 

 the beginning of things. Reliable calculations show that the years 

 required for the mere cooling of a globe like the sun could not 

 possibly run to millions. In other words, the sun's energy must 

 be subject to continuous and more or less steady renewal. How- 

 ever it may have acquired its enormous energy in the past, it must 

 have some source of energy in the present. 



The best explanation that we have to-day of this continuous 

 retion of energy is that it is due to shrinkage of the sun's bulk 

 under the force of gravity. Gravity is one of the most mysterious 

 forces of nature, but it is an obvious fact that bodies behave as 

 if they attracted one another, and Newton worked out the law of 

 this attraction. We may say, without trying to go too deeply into 

 things, that every particle of matter attracts every other through- 

 out the universe. I f the diameter of the sun were to shrink by one 

 mile all round, this would mean that all the millions of tons in the 



