Since, from the method of investigation, the temperatures of the eutectic curves will depend on the melting point of the third component (A), it is possible, by employing substances with widely differing melting points, to investigate the interaction of the two components (_e.g._ two optical antipodes) B and C over a range of temperature; and thus determine the range of stability of the compound, if one is formed. Since, in some cases, two substances which at one temperature form mixed crystals combine at another temperature to form a definite compound, the relationships which have just been described can be employed, and indeed, have been employed, to determine the temperature at which this change occurs.[334] By means of this method, Adriani found that below 103° _i_-camphoroxime exists as a racemic compound, while above {258} that temperature it occurs as a racemic mixed crystal[335] (_cf._ p. 219). B. Equilibria at Higher Temperatures. Formation of Double Salts.--After having studied the relationships which are found in the neighbourhood of the freezing points of the components, we now pass to the discussion of the equilibria which are met with at higher temperatures. In this connection we shall confine the discussion entirely to the systems formed of two salts and water, dealing more particularly with those cases in which the water is present in relatively large amount and acts as solvent. Further, in studying these systems, one restriction must be made, viz. that the single salts are salts either of the same base or of the same acid; or are, in other words, capable of yielding a common ion in solution. Such a restriction is necessary, because otherwise the system would be one not of three but of four components.[336] Transition Point.--As is very well known, there exist a number of hydrated salts which, on being heated, undergo apparent partial fusion; and in Chapter V. the behaviour of such hydrates was more fully studied in the light of the Phase Rule. Glauber's salt, or sodium sulphate decahydrate, for example, on being heated to a temperature of about 32.5°, partially liquefies, owing to the fact that the water of crystallization is split off and anhydrous sodium sulphate formed, as shown by the equation-- Na_{2}SO_{4},10H_{2}O = Na_{2}SO_{4} + 10H_{2}O The temperature of 32.5°, it was learned, constituted a _transition point_ for the decahydrate and anhydrous salt plus water; decomposition of the hydrated salt occurring above this temperature, combination of the anhydrous salt and water below it. Analogous phenomena are met with in systems constituted of two salts and water in which the formation of double salts can take place. Thus, for example, if _d_-sodium potassium {259} tartrate is heated to above 55°, apparent partial fusion occurs, and the two single salts, _d_-sodium tartrate and _d_-potassium tartrate, are deposited, the change which occurs being represented by the equation--