286 PHYSICAL SCIENCE 



hide our ignorance, but it is certain that many 

 changes quite beyond our range of terrestrial 

 experiment may be going on in the flaming 

 furnaces of the sun and stars. 



The problem of the probable age of the earth 

 is also surrounded with difficulty. The tempera- 

 ture of the earth rises as we pass underground, 

 and, from the present temperature gradient, 

 Lord Kelvin had calculated that about one 

 hundred million years ago the earth was a 

 molten mass. Although from the nature of the 

 assumptions made in this calculation, little 

 weight could be attached to the exact result 

 obtained, the estimated age of the earth, as the 

 home of organic life, was again too short for the 

 requirements of geology and biology. But it is 

 now known that radio-active matter in small 

 quantities is very widely distributed throughout 

 the earth and its atmosphere. Clay, for instance, 

 yields a radio-active emanation in appreciable 

 quantities, and potassium is slightly radio-active. 

 Rutherford calculated that, if all the substance 

 of the earth were as active as clay, the present 

 distribution of temperature might be maintained 

 by this cause alone. Such activity is unlikely, 

 but the result shows, at all events, that the 

 observed temperature gradient is not a safe 

 guide when used as the sole means of estimating 

 the age of the habitable globe. 



The great advance in knowledge, gained by 

 the study of the conduction of electricity through 

 gases and the phenomena of radiation and radio- 

 activity, cannot fail to exert a powerful influence 

 on the future of astro-physics, and, in particular, 



