PHYSICAL PARADOXES. 



arranged by going up a high mountain, where 

 there is so much less air overhead that its pres- 

 sure is much lower, and water boils so easily at 

 a low temperature that it cannot be made hot 

 enough to cook potatoes. 



Sulphur dioxide is a liquid which actually 

 boils at a temperature below that at which 

 water freezes. It boils at eighteen degrees of 

 frost. Liquid ammonia (not the cloudy house- 

 hold ammonia, which is water with some 

 ammonia dissolved in it, but pure liquid 

 ammonia) and liquid nitrous oxide boil at lower 

 temperatures still. 



Lower than most others is the temperature 

 at which liquid air boils. This substance, when 

 boiling, has 344 degrees of frost. 



Now, heat is always a condition of boiling. 

 Even at this low temperature, then, there can 

 be heat. It must not be supposed that there 

 is no heat at or below the temperature at which 

 water turns solid. We might as well say that 

 there is no heat below the temperature at 

 which melted wax or molten iron solidifies. 

 Heat is entirely relative. It means the vibra- 

 tion of molecules, and this vibration may be 

 more active or less active. When it is less 

 active, we say that the substance is colder, 

 which does not mean that it has no heat at all. 

 Every substance known to us has its molecules 

 vibrating, more or less energetically* that is, 

 it has some heat, more or less. 



172 



