igoS.J 



THE PHYSICS OF THE EARTH. 195 



surface. Even if the globe were a liquid mass of very small viscosity, 

 it is clear that such a separation of the elements could not take place. 



Finally it is to be recalled that recent experiments with radium 

 have shown the probable transmutation of some of the metals, as 

 when Sir Wm. Ramsay caused sulphate of copper to be partially 

 degraded into lithium. If this can occur for one or two metallic 

 elements, it may eventually be possible for many and perhaps all of 

 the metals. Our knowledge of these transformations is still in its 

 infancy, and we can not yet ascertain how minerals and metallic 

 veins have arisen ; but it is impossible to believe that the material 

 has come up from a pure supply at a depth of 1,500 kilometers. 

 It is much more probable that the metallic elements have been de- 

 veloped by differentiation and transformation from an original 

 magma, and that the whole interior of our planet is still a magma. 

 Differentiation of the elements appears to develop under conditions 

 met with in the crust, but now^here else. 



Accordingly we are obliged to dissent from the constitution of 

 the globe outlined by Professor W'iechert ; but in the matter of the 

 existence of a layer of plastic or fluid material just beneath the crust, 

 which he infers from the long seismic vibrations wath periods of 

 about eighteen seconds, we are in hearty accord with him. This is 

 definitely proved by the phenomena noticed in earthquakes, as more 

 fully set forth hereafter. It is the expulsion of lava from under the 

 margins of the sea which produces world-shaking earthquakes and 

 the upheaval of mountains along the sea coasts. 



§ 19. On Sir G. H. Darwin's Researches on the Stresses in the 

 Interior of the Earth Due to the Weight of Continents and Moun- 

 tains. — We have seen that the earth behaves as a solid at all depths, 

 unless it is in the thin layer just beneath the crust, in which move- 

 ments take place during earthquakes. The theory of an elastic 

 solid shows that when such a body is stressed the state of stress is 

 completely determined when the amount and direction of the three 

 principal stresses are known. Xo limit is imposed on these stresses 

 by theory, but in practice nature fixes a limit, beyond which the 

 elasticity breaks down, and the solid either flows or ruptures by 

 breaking. 



