444 OSMOTIC PPvESSURE. 



The freezing point of water is o°C., bnt if a solid is dissolved 

 in it, the freezing point is always below o°, the lowering being 

 up to a certain point proportional to the amount of solid dis- 

 solved. The material which separates on freezing is not the 

 solution of the solid, but pure water, provided the solution is 

 not too strong, so that it is quite correct to speak of the lower- 

 ing of the freezing point of the ivatcr by the presence of the 

 dissolved solid. It may be objected here that the law breaks 

 down with strong solutions, but this may be met by the fact 

 that water forms definite compounds with dissolved salts under 

 certain conditions of concentration and temperature, so that, 

 after a time, we no longer have the original salt in solution, 

 but a certain compound known as the cryohydrate, which, 

 being a body of definite composition, must have a definite 

 freezing point. Some still maintain that all cases of solution 

 result in the formation of definite hydrates. 



The boiling point of water under a pressure equal to that of 

 760 mm. of mercury is 100° C, but if a solid is dissolved in the 

 water, the boiling point is raised, and the rise is for dilute solu- 

 tions proportional to the amount of the dissolved solid. The 

 temperature of the steam though is still 100°. The easiest 

 method of treating the rise in the boiling point is by consider- 

 ing the difference in the vapour pressure of pure water and the 

 solution. 



The presence of dissolved matter in water reduces its vapour 

 pressure, and since no liquid can boil until the pressure of its 

 escaping vapour is equal to that of the atmosphere 

 above it, it follows that at 100° the vapour pressure of a solu- 

 tion must be less than that of pure water; but, as the vapour 

 pressure can be indefinitely increased by raising the tempera- 

 ture, it follows that the solution will boil on the temperature 

 being raised to some definite value, which will depend upon the 

 amount of solid in solution. Both the lowering of the freezing 

 point and the rise in the boiling point of water are examples of 

 perfectly general phenomena, which apply in the most diverse 

 cases. Thus the melting point of lead is lozvered by the 

 presence in it of silver, a metal which melts at a much higher 

 temperature, and the presence of a metal dissolved in mercury 

 raises the boiling point of the latter. There is a remarkable 

 relation between the properties of solutions and the molecular 

 weights of the dissolved substances. Equi-molecular solutions, 

 i.e., those containing the dissolved substances, in the propor- 

 tion of their molecular weights, in equal volumes of the salt 

 show the same depression of the freezing point, or the same 

 elevation of the boiling point, if the substances are similarly 

 constituted; in fact, so general is the relation, that boiling point 

 and freezing point determinations furnish the commonest 

 laboratory methods of controlling molecular weights, and until 

 the development of these methods by Raoult the molecular 

 weigh! of non-volatile bodies was always a matter of uncer- 

 tainty. 



Soluble solid substances may be divided into two classes, 

 electrolytric and non-electrolytric, accordingly as they conduct 



