THE EARTH'S INTERIOR HEAT. 303 



of the earth's crust increases at the rate of 1 for every 30 

 metres we descend from the surface towards the centre. Al- 

 though it is incredible that this augmentation can continue at 

 the same rate till the centre be reached, we may nevertheless 

 assume with certainty that it does continue to a considerable 

 depth. Calculation based on this assumption shows that at 

 a depth of a few miles a temperature must exist sufficiently 

 powerful to fuse most substances. Such molten masses pene- 

 trate the cold crust of the globe in many places, and make 

 their appearance as lava. 



A distinguished scientific man has lately expressed him- 

 self on the origin of the interior heat of the earth as follows : 

 " No one of course can explain the final causes of things. 

 This much, however, is clear to every thinking man, that 

 there is just as much reason that a body, like the earth, for 

 example, should be warm, warmer than ice or human blood, 

 as there is that it should be cold or colder than the latter. A 

 particular cause for this absolute heat is as little necessary as 

 a cause for motion or rest. Change that is to say, transi- 

 tion from one state of things to another alone requires and 

 admits of explanation." 



It is evident that this reflection is not fitted to suppress the 

 desire for an explanation of the phenomenon in question. As 

 all matter has the tendency to assume the same temperature 

 as that possessed by the substances by which it happens to be 

 surrounded, and to remain in a quiescent state as soon as 

 equilibrium has been established, we must conclude that, 

 whenever we meet with a body warmer than its neighbours, 

 such body must have received at a (relatively speaking) not 

 far distant time, a certain degree of heat, a process which 

 certainly allows of, and requires explanation. 



Newton's theory of gravitation, whilst it enables us to de- 

 termine, from its present form, the earth's state of aggrega- 

 tion in ages past, at the same time points out to us a source 

 of heat powerful enough to produce such a state of aggrega- 



