282 SCIENCE PROGRESS 



ture of the Earth is conditioned by the amount of heat received 

 from the Sun, the time during which the Earth can have 

 existed in its present state cannot be greater than the time 

 since which the temperature of the Sun has been about its 

 present value. Sir Ernest Rutherford has, however, shown 

 that even if the Sun were made entirely of uranium, only about 

 five million years would be added to its duration as a heat- 

 giver, so that here radioactivity can have but little effect. 

 Dr. Lindemann shows also that if the Sun were supposed 

 formed by the collision of two extinct stars, the duration of its 

 heat-supply would be unaffected by radioactivity. Therefore 

 neither radioactivity nor any other known cause will account 

 for a longer period than Kelvin supposed. 



To this argument C. E. Stromeyer replied {Nature, vol. 95, 

 p. 259, May 6) that there is a net loss in the Earth's heat over 

 and above any possible interchange of heat with the Sun, and 

 further that the permissible age of the Sun might be increased 

 by the heat-producing power of a meteoric bombardment. 

 Lindemann, however, pointed out (Nature, vol. 95, p. 372, 

 June 3) that the amount of energy lost in consequence of the 

 Earth's temperature gradient is 6,000 times less than the 

 total amount radiated, and so may be neglected, and, further, 

 that the heat due to meteoric bombardment to date had been 

 taken into account in the assumed mass of the Sun. 



A further objection was raised (Observatory, vol. 38, p. 261, 

 I 9 l S> J une )> viz- that, assuming stars to be formed by collision, 

 a short life of twenty million years for the Sun would require 

 the production of about fifty novae per year to maintain a 

 stellar system of one thousand million stars. Lindemann 

 replied (Observatory, vol. 38, p. 299, 191 5, July) that although 

 this number might be reduced owing to the fact that the real 

 age of the Earth may be considerably lengthened if the time is 

 included during which its temperature was much lower than 

 it is at present, yet the number of novae required still remains 

 larger than seems admissible. He suggests that this difficulty 

 may be avoided by admitting the existence of many dark 

 stars, which seems reasonable if the life of a single star is com- 

 paratively short. The strongest evidence against a short age 

 of the Sun and the Earth, in his opinion, is provided by Prof. 

 Strutt's estimation of the age of various radioactive minerals 

 from the amount of helium which they contain. He concludes 



