THE FIRE-HARDENED ROCKS 



is called Absolute Zero. But just as every gas becomes 

 a solid at that temperature, so there are temperatures at 

 which every solid becomes a gas. Gold, for instance, 

 begins to be a liquid at about 1900 F., and if we heat 

 it to 2000 it will become a gas. 



Therefore it will be seen that if we were to suppose 

 that the earth grew steadily hotter all the way down to 

 its centre, we should comparatively soon come to a point 

 when everything would be trying to turn into a gas. But 

 there is one other thing to be thought of. Imagine what 

 the pressure of the weight of the rocks themselves must 

 be. At a depth of a mile pressure from above arising from 

 the weight of the overlying rock is about 6000 Ib. to the 

 square inch. At three miles the weight has increased to 

 18,000 Ib., at four miles to about 24,000 Ib., and at five 

 miles to about 30,000 Ib. to the square inch. Now the 

 average strength required to crush rocks has been shown to 

 be about 25,000 Ib. to the square inch for granite, for 

 limestones about 16,000 Ib. to the square inch, and for the 

 sandstones about 6000 Ib. to the square inch. At a depth 

 of five miles, therefore, the weight above must be equal if 

 not greater than the resisting power of the rock. What 

 will happen lower than that ? An experiment shown 

 some years ago by Sir William Roberts Austen at the 

 Royal Institution gives us some idea of what might 

 happen. He subjected iron to very great hydraulic pres- 

 sure, and he arranged the experiment in such a way that 

 the spectators could see an image of what was happening 

 projected by a beam of light on to a kind of magic- 

 lantern screen. The iron began to move like slowly melt- 



85 



