803 NON-SATURATED VAPOUR. 



space than the quantity required to saturate it at the 

 actual temperature of the spaee, and which, therefore, 

 has a smaller tension and density than it would have at 

 the same temperature if the space were saturated. Such 

 vapour is called non-saturated, or c superheated/ because 

 its temperature is higher than that required to maintain 

 the same quantity as vapour, having exactly the same 

 tension. 



Non-saturated vapour exhibits the same kind of be- 

 haviour as a gas. Its tension increases in proportion 

 to the pressure upon it; it decreases the more it is 

 allowed to expand. It obeys in general Mariotte's law. 

 Gases are supposed to be the non-saturated vapours of 

 liquids which boil at temperatures more or less below 

 the ordinary temperature of the air. For since non- 

 saturated vapours have a smaller tension than the same 

 quantity of vapour might have at the same temperature, 

 it follows that those liquids, of which the non-saturated 

 vapours are gases at ordinary temperatures, could at 

 ordinary temperatures produce vapours of much higher 

 tension than the pressure under which they exist, that is, 

 the atmospheric pressure ; consequently, these vapours 

 must attain a tension of 760 mm at lower temperatures 

 than the ordinary ones, that is, their liquids must boil 

 at those lower temperatures. 



By compressing non-saturated vapour more and 

 more a point is finally reached when it saturates the space 

 which contains it, and if the pressure exerted upon it 

 is still further increased, it is reconverted into a liquid. 

 The same effect may be produced by lowering the 

 temperature sufficiently. Most gases may, indeed, be 

 i liquefied ' either by lowering their temperature or by 



