Reviews — Sir W. Thomson's Geological Dynamics. 475 



an end." There can be little doubt that the school which Professor 

 Huxley terms " Evolutionists " will eventually absorb both the 

 modern " Uniformitarians" and the older " Catastrophists." 



Part II. — On the Origin and Total Amount of Plutonic Energy. 



The store of energy to which the phenomena of volcanoes, earth- 

 quakes, and subsidences are due, is properly called plutonic energy. 



In modern physical dynamics the performance of work may be 

 described as the drawing of energy from one store and laying it out 

 elsewhere. Any irreversible transformation of energy is called a 

 dissipation of energy. 



Plutonic action is to be defined as any transformation of energy 

 going on within the earth, but it also involves something in the way 

 of dissipation of energy. The phenomena of volcanoes and earth- 

 quakes probably give rise to much less dissipation of energy than 

 the continual silent action of the conduction of heat outwards, the 

 amount of which may be estimated in a thoroughly definite manner. 

 It is found that from year to year the earth, at the present time, is 

 parting with heat at the rate of 92 horse-power^ per square kilometre.^ 



The store of energy (transformations of which constitute plutonic 

 action) consists almost entirely of terrestrial heat. This, indeed, is 

 the only description of energy proved to exist, in any considerable 

 quantity, within the earth ; but it is possible that there may be 

 great masses of uncombined chemical elements, and that the poten- 

 tial energy of their mutual affinities may constitute a considerable 

 portion of the plutonic energy in store, whether for the generation 

 of future underground heat, or for immediate application to some of 

 the more violent manifestations of plutonic activity. 



Sir W. Thomson remarks that we have strong reason to believe 

 the earth is not a mere thin shell filled with melted material of rock 

 or metal, but is solid from surface to centre, with the exception of 

 comparatively small spaces still occupied by fluid lava, or subjected 

 occasionally to melting in volcanic action.^ He concludes that it is 

 not at all probable that there is now within the earth a hundred 

 times as much heat as that which woidd raise a quantity of average 

 surface-rock equal in mass to the whole earth from zero to 200° 

 Cent,, since this would be certainly many times more than enough 

 to melt that amount of any kind of surface-rock under any moderate 

 pressure. 



Inasmuch as energy is being continually lost from the earth by 

 conduction through the upper strata, the whole quantity of plutonic 

 energy must have been greater in past times than at present. Sir 

 W. Thomson states that the only probable hypothesis in regard to 

 its origin is that the earth has become warm by the conversion of 

 mutual potential energy, whether of gravitational, or gravitational 

 and chemical, attraction between its parts, into heat. 



1 One horse-power is a rate of performing work equal to 33,000 foot-pounds per 

 minute. 



^ Kilometre = -62138 of a British statute mile. 



^ On this subject see the valuable papers in the Geological Magazine, by Mr. 

 G. Poulett Scrope, Vol. VI., p. 145 ; M. Delauny, Vol. V., p. 507; and Mr. D. 

 Forbes, Vol. IV., 1867. 



