THE STORY OF NINETEENTH-CENTURY SCIENCE 



stances. In the case of water, for example, it is a tem- 

 perature more than four hundred degrees above zero, 

 Centigrade; while for atmospheric air it is 194 Centi- 

 grade below zero, or more than a hundred and fifty de- 

 grees below the point at which mercury freezes. 



Be it high or low, the temperature above which any 

 substance is always a gas, regardless of pressure, is 

 called the critical temperature, or absolute boiling-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 tem- 

 perature the molecules have an enormous degree of ac- 

 tivity, and tend to fly asunder, maintaining what ap- 

 pears 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 degrees, however, 

 it is technically a vapor, not a gas ; but the sole diifer- 

 ence, it will be understood, is in the degree of molecular 

 activity. 



It thus appears 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 surface 

 temperature to be lowered to a degree which, in the 

 larger view, may be regarded as trifling. Between the 



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