THE LIFE OF THE SUN 15 



We must freely admit that this third argument has 

 not yet fully shared the fate of the two other lines of 

 reasoning. Indeed, Professor Sir George Darwin, 

 although not feeling the force of these latter, agrees 

 with Lord Kelvin in regarding 500 million years as 

 the maximum life of the sun. 1 



We may observe, too, that 500 million years is by no 

 means to be despised : a great deal may happen in such 

 a period of time. Although I should be sorry to say 

 that it is sufficient, it is a very different offer from 

 Professor Tait's ten million. 2 



In drawing up this account of the physical arguments, 

 I owe almost everything to Professor Perry for his 

 articles in Nature (January 3 and April 18, 1895), and 

 his kindness in explaining any difficulties that arose. 

 I have thought it right to enter into these arguments in 

 some detail, and to consume a considerable proportion 

 of our time in their discussion. This was imperatively 

 necessary, because they claimed to stand as barriers across 

 our path, and, so long as they were admitted to be 

 impassable, any further progress was out of the question. 

 What I hope has been an unbiased examination has 

 shown that, as barriers, they are more imposing than 



1 British Association Reports, 1886, pp. 514-18. 



2 Professor Perry has kindly sent me a few lines on the bearing of 

 the discovery of Radium upon the problem. ' At the time when your 

 address was delivered,' he writes, 'I thought that the sun's heat argument 

 was the one that was most difficult to meet. But now, the discovery of 

 Radium has disposed of it as well as the conclusions founded on 

 conductivity. The duration of Radium itself is known to be only a few 

 thousand years, but quantity of Radium indicates quantity of that substance, 

 probably Uranium, whose exceedingly slow change is constantly producing 

 Radium. 



' 1. The heat conductivity argument. This is completely disposed of 

 even if Mr. Strutt has overestimated the amount of Radium in rock. 

 Suppose only ^th of what he has assumed from his measurements and we 

 have the possibility of multiplying Lord Kelvin's age by 1,000 or more. 



' 2. The sun's heat argument. Assume that there is Radium in the sun 

 and this gives us almost any multiple we please to imagine of the total 

 energy assumed by Helmholtz. We are now in a position to say that 

 the physicist can make no calculation either as to the probable or possible 

 age of life on the earth.' Nov. 8, 1906. 



