------------------------------------------------------------------ Salt. | Cryohydric point. | Percentage of anhydrous | | salt in the cryohydrate. ------------------------------------------------------------------ Sodium bromide | -24° | 41.33 Sodium chloride | -22° | 23.60 Potassium iodide | -22° | 52.07 Sodium nitrate | -17.5° | 40.80 Ammonium sulphate | -17° | 41.70 Ammonium chloride | -15° | 19.27 Sodium iodide | -15° | 59.45 Potassium bromide | -13° | 32.15 Potassium chloride | -11.4° | 20.03 Magnesium sulphate | -5° | 21.86 Potassium nitrate | -2.6° | 11.20 Sodium sulphate | -0.7° | 4.55 ------------------------------------------------------------------ {119} The chemical individuality of these cryohydrates was, however, called in question by Pfaundler,[198] and disproved by Offer,[199] who showed that in spite of the constancy of the melting point, the cryohydrates had the properties, not of definite chemical compounds, but of mixtures; the arguments given being that the heat of solution and the specific volume are the same for the cryohydrate as for a mixture of ice and salt of the same composition; and it was further shown that the cryohydrate had not a definite crystalline form, but separated out as an opaque mass containing the two components in close juxtaposition. The heterogeneous nature of cryohydrates can also be shown by a microscopical examination. At the cryohydric point, therefore, we are not dealing with a single solid phase, but with two solid phases, ice and salt; the cryohydric point, therefore, as already stated, is a quadruple point and represents an invariant system. Although on cooling a solution to the cryohydric point, separation of ice may occur, it will not necessarily take place; the system may become metastable. Similarly, separation of salt may not take place immediately the cryohydric point is reached. It will, therefore, be possible to follow the curves BO and AO beyond the quadruple point,[200] which is thereby clearly seen to be the point of intersection of the solubility curve of the salt and the freezing-point curve of ice. At this point, also, the curves of the univariant systems ice--salt--vapour and ice--salt--solution intersect.