248 MOREY 



ART. G 



curve again has a positive slope. In a system of the type, 

 H2O-KNO3, the experimental realization of this portion of the 

 curve would be extremely difficult and we will not consider it 

 further at present, except to point out that at zero water content 

 the equation becomes 



dp 77* — rj' 



dt V — v^ 



which is the equation of the tangent to the melting-point curve 

 of pure KNO3. The p-t curve of the saturated solutions is 

 therefore tangent at its end to the melting-point curve of 

 KNO3, the curve showing the change in melting point of 

 potassium nitrate with pressure. This type of equilibrium will 

 be considered later. 



9. The Second Boiling Point. We have seen that a melting- 

 point or solubility curve of the system, H2O-KNO3, extends 

 from the cryohydrate E to the melting point of pure KNO3, 

 and have followed the change in vapor pressure with composi- 

 tion in detail. We have therefore correlated the temperature- 

 composition or solubility curve with the pressure-temperature 

 curve. One curve gives the change with the temperature in the 

 composition of the liquid in equilibrium with solid and vapor, the 

 other gives the change with temperature in the vapor pressure 

 of the saturated solution. One other pair of the three vari- 

 ables, composition of the liquid, temperature, and pressure, 

 can be considered, namely, the change in vapor pressure of the 

 saturated solution with composition. This is the pressure- 

 composition curve; from it we see that the vapor pressure at 

 first increases with decreasing water content of the saturated 

 solutions, reaches a maximum at a small H2O content, then 

 decreases rapidly with further diminution of the water content, 

 until at its end-point at pure KNO3 the vapor pressure is that 

 of the triple point of KNO3. We are all familiar with the fact 

 that as the water content of the saturated solution decreases 

 with increasing temperature the vapor pressure increases, until 

 at the boiling point of the solution the pressure of the atmos- 

 phere is reached. But there are two saturated solutions whose 

 vapor pressure is one atmosphere; one has a water content of 29 



