x] RADIO-ACTIVE PROCESSES 343 
of radium emits 100 gram-calories per hour, and on the observa- 
tion of Langley that each square centimetre of the sun’s surface 
emits 8°28 x 10° gram-calories per hour. Since the average density 
of the sun is 1°44, the presence of radium in the sun, to the 
extent of 2°5 parts by weight in a million, would account for its 
present rate of emission of energy. 
An examination of the spectrum of the sun has not so far 
revealed any of the radium lines. It is known, however, from 
spectroscopic evidence that helium is present, and this indirectly 
suggests the existence of radio-active matter also. It can readily 
be shown? that the absence of penetrating rays from the sun at 
the surface of the earth does not imply that the radio-elements 
are not present in the sun. Even if the sun were composed of 
pure radium, it would hardly be expected that the y rays emitted 
would be appreciable at the surface of the earth, since the rays 
would be almost completely absorbed in passing through the 
atmosphere, which corresponds to a thickness of 76 centimetres of 
mercury. 
In the Appendix E of Thomson and Tait’s Natural Philosophy, 
Lord Kelvin has calculated the energy lost in the concentration of 
the sun from a condition of infinite dispersion, and concludes that 
it seems “on the whole probable that the sun has not illuminated 
the earth for 100,000,000 years and almost certain that he has not 
done so for 500,000,000 years. As for the future we may say, with 
equal certainty, that inhabitants of the earth cannot continue to 
enjoy the light and heat essential to their life for many million 
years longer, unless sources now unknown to us are prepared in 
the great storehouses of creation.” 
The discovery that a small mass of a substance like radium 
can emit spontaneously an enormous quantity of heat renders 
it possible that this estimate of the age of the sun’s heat 
may be much increased. In a letter to Nature (Sept. 24, 1903) 
G. H. Darwin drew attention to this probability, and stated that, 
“The lost energy of the sun, supposed to be a homogeneous sphere 
of mass M and radius a, is 34M?/a where pw is the constant of 
gravitation. On introducing numerical values for the symbols in 
this formula, I find the lost energy to be 2°7 x 10’ M calories where 
1 See Strutt and Joly, Nature, Oct. 15, 1903. 
