Solubility Curve at Higher Temperatures.--On passing to the consideration of the solubility curves at higher temperatures, two chief cases must be distinguished. (1) The two components in the fused state can mix in all proportions. (2) The two components in the fused state cannot mix in all proportions. 1. _Complete Miscibility of the Fused Components._ [Illustration: FIG. 28.] The best example of this which has been studied, so far as anhydrous salts and water are concerned, is that of silver nitrate and water. The solubility of this salt at temperatures {115} above 100° has been studied chiefly by Etard[190] and by Tilden and Shenstone.[191] The values obtained by Etard are given in the following table, and represented graphically in Fig. 28. SOLUBILITY OF SILVER NITRATE. --------------------------------------------------- Temperature. |Parts of dry salt in 100 parts | of solution. --------------------+------------------------------ -7° | 46.2 -1° | 52.1 +5° | 56.3 10° | 61.2 20° | 67.8 40.5° | 76.8 73° | 84.0 135° | 92.8 182° | 96.9 --------------------------------------------------- In this figure the composition of the solution is expressed in parts of silver nitrate in 100 parts by weight of the solution, so that 100 per cent. represents pure silver nitrate. As can be seen, the solubility increases with the temperature. At a temperature of about 160° there should be a break in the curve due to change of crystalline form (p. 63). Such a change in the direction of the solubility curve, however, does not in any way alter the essential nature of the relationships discussed here, and may for the present be left out of account. On following the solubility curve of silver nitrate to higher temperatures, therefore, the concentration of silver nitrate in the solution gradually increases, until at last, at a temperature of 208°,[192] the melting point of pure silver nitrate is reached, and the concentration of the water has become zero. The curve throughout its whole extent represents the equilibrium between silver nitrate, solution, and vapour. Conversely, starting with pure silver nitrate in contact with the fused salt, addition of water will lower the melting point, _i.e._ will lower the temperature at which the solid salt can exist in contact with the liquid; {116} and the depression will be all the greater the larger the amount of water added. As the concentration of the water in the liquid phase is increased, therefore, the system will pass back along the curve from higher to lower temperatures, and from greater to smaller concentrations of silver nitrate in the liquid phase. The curve in Fig.