GENESIS OF THE ELEMENTS 160 



the waters from the dry land, and in fact the establishment of 

 conditions which would fit it to be the abode of life ? 



Attempts to answer this question have assumed a variety of 

 forms, and very diverse estimates have been arrived at. 



From inferences as to the rate of cooling of the earth deduced 

 from observations of the temperatures at different depths in the 

 crust Lord Kelvin estimated that not more than one hundred 

 million years were required for the earth to cool down from the 

 temperature when in the molten state to its present state. 



Many geologists and physicists have studied the question 

 and have arrived at different conclusions. The History of the 

 Geological Society of London, published in 1907, on the occasion 

 of the Centenary of the Society, contains the following passage 

 (p. 227) : "At present it may be said that an estimate of one 

 hundred million years, for the period during which life has 

 existed on the earth, is regarded as fairly approximate." Such 

 is the view of the geologists, but as all such estimates are based 

 on very insecure foundations they can be regarded only as mere 

 speculations which serve to show how incomplete is our know- 

 ledge of the world in which we live. 



The properties of radium have provided a new starting-point 

 which, however, cannot be regarded as more secure than the 

 rest. Radio-active matter is found everywhere in the rocks of 

 the earth's crust as well as in the ocean and the gases of the 

 atmosphere. The estimation of the amount of radium in the 

 rocks is beset with difficulties, and it is certain that the amount 

 varies enormously from rock to rock, being probably about 

 fifteen times as much in some of the old granites as in basalts 

 and some other rocks which are presumably less ancient. 



Radium in its process of spontaneous disintegration gives out 

 heat continually, and this heat developed in the substance of 

 the earth might counteract the effect of radiation by which the 

 earth is continually parting with heat and tending to cool down. 



Now it has been estimated by Professor Rutherford that if 

 one gram of radium emits 100 calories per hour " the presence of 

 4-6 xlO~ 14 gram of radium per gram uniformly distributed 

 throughout the volume of the earth would produce as much heat 

 as that lost from the earth by conduction to the surface. In 

 other words, with such a distribution of radium, the temperature 

 gradient of the earth would remain constant." 



Now the average amount of radium found in rocks is about 



