PHYSICAL PARADOXES. 



acquainted have some heat, though those which 

 we call cold have less than those which we call 

 hot. To make ice colder, then, we must con- 

 trive to let it get rid of some of its heat. 



Now, heat is a form of energy, transform- 

 able into many other forms of energy, such as 

 light, electricity, magnetism, combination, de- 

 composition, liquefaction, vaporisation, pressure, 

 and movement. We see this in our steam 

 engines and gas engines, which are merely 

 machines for converting heat energy into move- 

 ment energy. To deprive a thing of heat, 

 therefore, is to withdraw energy from it, leaving 

 it exhausted, or more exhausted than before, of 

 energy. How, then, can we get things ex- 

 hausted of energy ? 



The principle is the same for inanimate 

 things as for men and other animals. Exertion 

 brings exhaustion. If the doctor recommends 

 us for our health to get quite tired once or twice 

 a day, what we do is to work hard. It is true 

 that some people get tired before they have done 

 much hard work, but the principle remains true 

 for the rest of us, and it applies equally to things. 



Among other kinds of work, heat can do 

 melting, whether of iron, lead, zinc, tallow, or 

 wax. Let the heat of the ice then do some 

 melting, and, for convenience, let it melt some 

 of the ice itself. 



It is not possible, however, for the heat of 

 pure ice to melt ice, since that heat is not at a 



118 



