THE FIRE-HARDENED ROCKS 



would be perhaps 5000 F. ; a temperature at which 

 steel would melt and boil away into vapour. At a depth 

 of 200 miles the heat would be greater than that of the 

 surface of the sun. 1 



Now at temperatures like that everything we know 

 on the surface of the earth would melt. Something else 

 would happen to it besides that. Those of our readers 

 who have ever seen experiments at the Royal Institution 

 in London by Sir James Dewar or Sir William Crookes 

 will know that if metals are made hot enough they will 

 not only melt but will boil away into vapour as water 

 boils into steam. And perhaps we need tell no one that 

 air, if it be subjected to a low enough temperature, can be 

 made a solid like ice. In fact, everything in nature, 

 whether we generally know it as a solid (like iron), 

 or a liquid (like water), or a gas (like air), can be 

 made to assume either of the two other forms. Thus 

 the solid iron can be turned into a liquid or a gas, 

 and the liquid water can be turned into a gas by boil- 

 ing, or into ice by freezing. The gaseous air can be 

 turned into a liquid by lowering its temperature to some 

 300 F. or more below the point at which water turns into 

 ice ; while if we lower the temperature to about 390 F. 

 below freezing, it will turn into a solid. At a temperature 

 of about 490 F. below freezing everything in nature, 

 whether gaseous or liquid, would become a solid, and that 

 temperature, which is the lowest that can possibly exist, 



1 According to the calculations made by the late Mr. W. E. 

 Wilson, F.R.S., in Ireland, 5773 Centigrade above the lowest temper- 

 ature which is possible in space, or about 10,500 F. 



84 



