lOO PHYSICAL SCIENCE 



osmotic pressure may be made to do work, which 

 will be measured by the pressure multiplied by the 

 change of volume. Thus the osmotic pressure is 

 measured by the change of the available energy 

 per unit increase of volume ; that is, by the rate 

 of change in the available energy of dilution. 



In this manner we arrive again at the con- 

 clusion, that the osmotic pressure must be equal 

 in amount to the gaseous pressure exerted by 

 the same number of molecules when vaporised, 

 and must conform to the laws which describe 

 the temperature, pressure, and volume relations 

 of gaseous matter. The result is seen clearly to 

 be independent of any hypothesis concerning the 

 mechanism of the pressure or the nature of the 

 solution. 



In the last chapter we have traced the 

 phenomena of fusion and solidification, and, in 

 the course of our inquiry, studied the equilibrium 

 of liquid solutions with the different solid phases 

 which may exist in contact with the liquids. The 

 fundamental problem of the nature of a solution 

 was untouched ; indeed, from the point of view 

 then adopted, such a problem did not arise. 



Until the last quarter of the nineteenth century, 

 it was generally assumed that the forces which 

 were brought into play when a solid dissolved in 

 water were of the same nature as those involved 

 in chemical action ; and the resulting solution 

 was looked on simply as a chemical compound 

 in which there happened to be no fixed relation 

 between the masses of the components. The study 

 of dilute solutions, and, in particular, the examina- 

 tion of their osmotic pressures, showed that, in 

 many respects, a dilute solution was analogous 



