AGE OF THE EARTH EAYLEIGH. 251 



cally not. It appears certain that the radioactive materials present 

 in the earth are generating at least as much heat as is now leaking out 

 from the earth into space. If they are generating more than this 

 (and there is evidence to suggest that they are) , the temperature must, 

 according to all received views, be rising. In a word, we are puzzled 

 to explain the existing state of things, and cannot use it as a firm 

 basis from which to explore the past. 



Next, as to the sun's heat. Lord Kelvin's argument was that we 

 knew of no possible source at all adequate to supply the existing out- 

 put of solar energy except secular contraction, and even this source 

 of supply was not enough to account for more than 20,000,000 years 

 of solar heat in the past. It is impossible to condemn on principle 

 arguments of this kind. We often must, and do, rely on them in 

 science as in everyday affairs ; but a certain reserve is always needed 

 on the ground that there are more things in heaven and earth than 

 are dreamt of in our philosophy. Knowledge which has accumulated 

 since Lord Kelvin's time has driven us back on this alternative. 



The sun is only one of the host of stars, and if we find it impossible 

 to account adequately for their radiation by contraction it evidently 

 will not do to assume that the sun is limited to this source of supply. 



Now, some of the stars (the giant red stars), though of about the 

 same mass as the sun, are radiating energy at something like one 

 thousand times the rate that the sun does. They ought, according to 

 the contraction theory, to have expended a considerable fraction of 

 their total energy in historical times. No one will maintain that this 

 has occurred, and if not there must be some source of supply other 

 than contraction. It is not necessary for our immediate purpose to 

 inquire what this source is. It is enough to note that its existence 

 invalidates Lord Kelvin's estimate of the age of the sun's heat. 



Modern knowledge in radioactivity has given what appears, if 

 separately considered, to be a firm and satisfactory basis for the 

 estimation of geological time. Uranium, for example, goes through 

 a series of changes (radium is one of the stages in its progress) , chang- 

 ing eventually into an isotope of lead — that is, an element chemically 

 indistinguishable from lead, except by a slight difference of atomic 

 weight and (practically at least) inseparable from ordinary lead by 

 chemical means if once mixed with it. The isotope of lead in question 

 has probably an atomic weight of 206 exactly, as contrasted with an 

 atomic weight of 207.1 for ordinary lead. 2 This is much less than 

 the atomic weight of uranium (238.5), and the difference represents 

 approximately the weight of helium atoms, which are the debris shed 

 at the various stages of the transformation. 



2 Ordinary lead may partly consist of it, but this is not yet certain, and not very 

 important for the immediate purpose. 



