SOLUTIONS 2 1 



atmosphere. The addition of a solute such as sugar, whose 

 solution is not ionizable, and therefore does not conduct 

 electricity, lowers the vapour tension of the solution in 

 proportion to the molecular concentration of the solute. 



Corresponding Values. — We have thus found five properties 

 of a solution which vary proportionally, so that from the 

 measurement of any one of them we can determine the 

 corresponding values of all the others. These are — 



1. The Molecular Concentration. 



2. The Osmotic Pressure. 



3. The Diminution of Vapour Tension. 



4. The Raising of the Boiling Point. 



5. The Lowering of the Freezing Point. 



Cryoscopy. — The usual method employed for the deter- 

 mination of the molecular concentration and osmotic 

 pressure of a solution is by cryoscopy — the measurement of 

 its temperature of congelation. A very sensitive thermometer 

 is used, the scale of which extends over only 5" and is divided 

 into hundredths of a degree. The liquid under examination is 

 placed in a test tube, in which the bull) of the thermometer 

 is plunged, and this is supported in a second tube with an air 

 space all round it. The whole is then suspended to the under 

 side of the cover of the refrigerating vessel, which may be 

 cooled either by filling it with a freezing mixture, or by the 

 evaporation of ether. During the whole of the operation the 

 liquid is agitated by a mechanical stirrer. The first step is 

 to determine the freezing point of distilled water. As the 

 water cools the mercury gradually descends in the stem of 

 the thermometer till it reaches a point below the zero mark 

 at 0° C. As soon as ice begins to form the mercury rises, at 

 first rapidly and then more slowly, reaches a maximum, and 

 finally descends again. This maximum reading is the true 

 point of congelation. The inner tube is then emptied, care 

 being taken to leave a few small ice crystals to serve as centres 

 of congelation for the subsequent experiment, thus avoiding 

 supercooling of the solution. The process is then repeated 

 with the solution under examination. The difference between 



