equilibrium was controlled by determinations of the density of the solution. SOLUBILITY OF AMMONIUM NITRATE. ------------------------------------------------------------ Temperature. | Solubility. | Temperature. | Solubility. --------------+-------------+--------------+---------------- 12.2° | 34.50 | 32.7° | 57.90 20.2° | 43.30 | 34.0° | 58.89 25.05° | 48.19 | 35.0° | 59.80 28.0° | 51.86 | 36.0° | 61.00 30.0° | 54.40 | 37.5° | 62.90 30.2° | 54.61 | 38.0° | 63.60 31.9° | 57.20 | 39.0° | 65.09 32.1° | 57.60 | 40.0° | 66.80 ------------------------------------------------------------ From the graphic representation of the solubility given in Fig. 27, there is seen to be a distinct change in the direction of the curve at a temperature of 32°; and this break in the curve corresponds to the transition of the [beta]-rhombic into the [alpha]-rhombic form of ammonium nitrate (p. 63). Suspended Transformation and Supersaturation.--As has already been learned, the transformation of the one crystalline form into the other does not necessarily take place immediately the transition point has been passed; and it has therefore been found possible in a number of cases to follow the solubility curve of a given crystalline form beyond the point at which it ceases to be the most stable modification. Now, it will be readily seen from Fig. 27 that if the two solubility curves be prolonged beyond the point of intersection, the solubility of the less stable form is greater than that of the more stable. A solution, therefore, which is saturated with respect to the less stable form, _i.e._ which is in equilibrium with that form, is _supersaturated with respect to the more stable modification_. If, {114} therefore, a small quantity of the more stable form is introduced into the solution, the latter must deposit such an amount of the more stable form that the concentration of the solution corresponds to the solubility of the stable form at the particular temperature. Since, however, the solution is now _unsaturated_ with respect to the less stable variety, the latter, if present, must pass into solution; and the two processes, deposition of the stable and solution of the metastable form, must go on until the latter form has entirely disappeared and a saturated solution of the stable form is obtained. There will thus be a conversion, through the medium of the solvent, of the less stable into the more stable modification. This behaviour is of practical importance in the determination of transition points (_v._ Appendix). From the above discussion it will be seen how important is the statement of the solid phase for the definition of saturation and supersaturation.[189]