172 THE REALITIES OF MODERN SCIENCE 



of matter, we see that what we have been accustomed 

 to consider solid substances are merely substances for 

 which the melting point (i.e. the freezing point) is well 

 above the temperatures which we meet under our cli- 

 matic conditions. The characteristic of being solid 

 is not one of the substance itself but is dependent upon 

 the physical conditions of pressure and temperature 

 under which it is at the time. Similar statements 

 may be made as to substances which we are accustomed 

 to consider liquid. Substances which we know as 

 gases are of course those for which the boiling point 

 (i.e. the liquefaction point) is well below ordinary 

 temperatures. 



