ETHER AND PONDERABLE MATTER 



point, of that substance. It does not follow, however, 

 that below this point the substance is necessarily a 

 liquid. This is a matter that will be determined by 

 external conditions of pressure. Even far below the 

 critical temperature the molecules have an enormous 

 degree of activity, and tend to fly asunder, maintaining 

 what appears to be a gaseous, but what technically is 

 called a vaporous, condition the distinction being that 

 pressure alone suffices to reduce the vapor to the liquid 

 state. Thus water may change from the gaseous to 

 the liquid state at four hundred degrees above zero, 

 but under conditions of ordinary atmospheric pressure 

 it does not do so until the temperature is lowered three 

 hundred degrees further. Below four hundred de- 

 grees, however, it is technically a vapor, not a gas ; but 

 the sole difference, it will be understood, is in the de- 

 gree of molecular activity. 



It thus appeared that the prevalence of water in a 

 vaporous and liquid rather than in a "permanently" 

 gaseous condition here on the globe is a mere incident 

 of telluric evolution. Equally incidental is the fact 

 that the air we breathe is "permanently" gaseous and 

 not liquid or solid, as it might be were the earth's sur- 

 face temperature to be lowered to a degree which, in 

 the larger view, may be regarded as trifling. Between 

 the atmospheric temperature in tropical and in arctic 

 regions there is often a variation of more than one hun- 

 dred degrees; were the temperature reduced another 

 hundred, the point would be reached at which oxygen 

 gas becomes a vapor, and under increased pressure 

 would be a liquid. Thirty-seven degrees more would 

 bring us to the critical temperature of nitrogen. 



303 



