HETEROGENEOUS EQUILIBRIUM 245 



decreases; that of the sohd increases, as does that of the hquid. 

 The first term in the numerator consequently decreases, the 

 second increases, and the coefficient of the second term also 

 increases; since the first term is positive, while the second is 

 negative, the numerator is a continually decreasing positive 

 quantity. The denominator is decreasing at a progressively 

 slower rate. As the temperature is raised these effects con- 

 tinue, until a temperature is reached at which the rate of 

 decrease of the numerator becomes equal to that of the denomi- 

 nator, and the curve has a point of inflection. After this it is 

 no longer concave upward, but is concave downward, as the 

 vapor pressure of the saturated solution is still increasing with 

 the temperature, but at a diminishing rate. The temperature 

 of this point of inflection is approximately 205°, and the pres- 

 sure is about 5.3 atmospheres. 



The determination of the solubility curve of KNO3 in HoO 

 is a simple matter at temperatures below 100°. As long as the 

 vapor pressure remains less than one atmosphere, we can shake 

 up solid and liquid in a thermostat until equilibrium is reached, 

 suck out a sample of the supernatant liquid through a filter, 

 and determine the composition by analysis. After the pressure 

 has exceeded one atmosphere, other methods must be employed. 

 Of course, if a mixture containing an excess of KNO3 is heated 

 in an open vessel, when the vapor pressure reaches one atmos- 

 phere the solution will begin to boil, and will evaporate to 

 dryness. But if the mixture be heated in a closed tube, from 

 which the water cannot evaporate, the solubility curve will be 

 continuous until the mixture is entirely liquid ; the temperature 

 at which the saturated solution boils at a pressure of one 

 atmosphere is not a significant point on the solubility curve. 

 From this point of view there is no distinction between a 

 solubility curve and a melting-point curve, and the curve EBm 

 can be regarded either as the solubility curve of KNO3 in H2O 

 or as the melting-point curve of H2O-KNO3 mixtures. The 

 first to realize this fact was Guthrie* in 1884, and the system, 

 H2O-KNO3, was one of those that he studied. He sealed 



* Guthrie, Phil. Mag., 18, 117 (1884). 



