HISTORY OF THE CONSERVATION OF ENERGY m 



3 X 10" 12 grains of radium as the average amount present in 1 c.e. 

 of soil. 



From the rate of increase of temperature below the earth's surface 

 and the heat conductibility of rocks, Mr. Strutt concludes that radium 

 is confined to a comparatively thin crust of the earth. While these 

 reasons are not conclusive, they are weighty. Our incomplete knowl- 

 edge of the properties and the distribution of radium and other radio- 

 active substances, makes it necessary to suspend judgment on the age 

 of the earth. There is no necessity that the question be settled 

 immediately. 



The same remark applies to the antiquity of the sun. Much de- 

 pends upon the presence or absence of radium there. As yet this sub- 

 stance has not been found in the sun, but the presence of helium, com- 

 bined with the fact that helium may be obtained from radium, renders 

 the presence of radium in the sun quite probable. That radium affects 

 the problem of the solar age was pointed out by Mr. G. H. Darwin in 

 the following words : 46 



Knowing, as we now do, that an atom of matter is capable of containing 

 an enormous store of energy in itself, I think we have no right to assume that 

 the sun is incapable of liberating atomic energy to a degree at least com- 

 parable with that which it would do if made of radium. Accordingly, I see no 

 reason for doubting the possibility of augmenting the estimate of solar heat as 

 derived from the theory of gravitation by some such factor as ten or twenty. 



In conclusion, it is very evident that, however unpleasant it may 

 be for the older men to revise their theories to meet the demands of 

 new observations, we have in radio-activity the entrance into a region 

 of new knowledge which will cast light upon many a dingy avenue of 

 philosophy. Great are the trials and great the final triumphs of experi- 

 mental science. 



In Norse m} T thology there is a wonderful tree called Igdrasil, whose 

 branches spread over the whole earth and reach up into the clouds. 

 At the foot of the tree, away down at the deepest root, is a well from 

 which the tree draws its sap. To us of the twentieth century that 

 tree symbolizes science. The well which nourishes the tree is the 

 fountain of eternal truth. 



46 Nature, Vol. 68, 1903, p. 496. 



V 



