Page:Text-book of Electrochemistry.djvu/56

 \ X

��(BAPTER IV. Vapour Pressure of Solutions.

The Vapour Pressure of a Solution is lower than that of the Pure Solvent. — It has been well known for a long time that a solution in water of a substance which has no appreciable vapouv pressure has a lower vapour tension than water. Thus, for instance, the vapour tension of water can be reduced by the addition of sulphuric acid, and such sohitions are, therefore, used to extract the moisture from the air. If the vapour tension is lowered, the boiling point must be raised, because when a liquid boils, its vapour pressinre amounts to 1 atmo., and consequently, when the vapour pressure is diminished by the addition of some foreign substance, the temperature must be raised in order that the pressure may reach the value of 1 atmo.

This corollary has also been known for a long time ; for instance, if a salt be dissolved in water, the boiling point of the solution (at 760 mm. Hg) is higher than 100"^, and the more salt is added the higher is the boiling point. (Faraday, 1822 ; Legrand, 1833.)

Connection between Vapour Pressure and Osmotic Pressure of a Solution. — In the very first investigation made on osmotic pressure, it was found that the depression of the vapour pressure of a solution was almost exactly pro- l)ortional to the osmotic pressure. A conclusive proof of this proportionality, based on the mechanical theory of heat, was tirst deduced by van't Hoff {!) in a paper to the Swedish Academy (1886). I (?) have later deduced the same

��N

�� �