Evaporation and Dissociation. 



453 



mined vapour-pressure ; and by means of a planimeter (by- 

 Stanley of Holborn) the areas enclosed between each hori- 

 zontal line and the curves respectively below and above it, 

 formed by the isothermal lines, were measured. To ascertain 

 what position of the horizontal line would render these areas 

 equal, the values of each set of three areas were mapped on 

 sectional paper as abscissae, the pressures corresponding to the 

 position of the horizontal lines being ordinates. The curves 

 passing through the resulting points cut each other at a point 

 which represents the true pressure and the true area. This 

 method is rendered clearer by inspection of the following 

 figure : — 













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It is evident that these vapour-pressures depend entirely on 

 measurements represented in the diagram outside the area 

 bounded by the curve representing orthobasic volumes of gas 

 and liquid. It will be seen, on reference to the table on p. 444, 

 that the agreement between these calculated vapour-pressures 

 and those experimentally determined is a very close one, the 

 greatest difference being about 1 per cent. This agreement 

 between experimentally observed vapour-pressures and those 

 depending on the formula p = bt—a is very remarkable, and 

 it is difficult to believe that, if the isochoric lines were curves, 

 such an agreement could exist. 



What are usually termed vapour-pressures, then, are those 

 pressures at which horizontal lines drawn through them render 

 the areas enclosed by the isothermal lines below the horizontal 

 lines equal to those above them. But there are other two 

 conditions of matter, each of which has its characteristic 

 pressures. One of these is represented by the highest pressure 

 attainable on any isothermal, or the summit of the curve above 

 the vapour-pressure line ; and the other the apex of the curve 

 below the vapour-pressure line. Each temperature chosen has 



