contact with its saturated solution at the transition point; or, in other words, the racemate is not decomposed by water at the transition point. The same behaviour will evidently be exhibited by sodium ammonium racemate at 27°, for the two enantiomorphous sodium ammonium tartrates have also identical solubility. {268} Very different, however, is the behaviour of, say, astracanite, or of the majority of double salts; for the solubility of the constituent salts is now no longer the same. If, for example, excess of a mixture of sodium sulphate and magnesium sulphate, in equimolecular proportions, is brought in contact with water below the transition point (22°), more magnesium sulphate than sodium sulphate will dissolve, the solubility of these two salts in a common solution being given by the following figures, which express number of molecules of the salt in 100 molecules of water.[351] COMPOSITION OF SOLUTIONS SATURATED WITH RESPECT TO Na_{2}SO_{4},10H_{2}O AND MgSO_{4},7H_{2}O. ---------------------------------------- Temperature. | Na_{2}SO_{4}. | MgSO_{4}. ---------------------------------------- 18.5° | 2.16 | 4.57 24.5° | 3.43 | 4.68 ---------------------------------------- At the transition point, then, it is evident that the solution contains more magnesium sulphate than sodium sulphate: and this must still be the case when astracanite, which contains sodium sulphate and magnesium sulphate in equimolecular proportions, separates out. If, therefore, the temperature is raised slightly above the transition point, magnesium sulphate and sodium sulphate will pass into solution, the former, however, in larger quantities than the latter, and astracanite will be deposited; and this will go on until all the magnesium sulphate has disappeared, and a mixture of astracanite and sodium sulphate decahydrate is left as solid phases. Since there are now three phases present, the system is univariant (by reason of the restriction previously made that the vapour phase is absent), and at a given temperature the solution will have a definite composition; as given in the following table:-- COMPOSITION OF SOLUTIONS SATURATED WITH RESPECT TO Na_{2}Mg(SO_{4})_{2},4H_{2}O AND Na_{2}SO_{4},10H_{2}O. ---------------------------------------- Temperature. | Na_{2}SO_{4}. | MgSO_{4}. ---------------------------------------- 22° | 2.95 | 4.70 24.5° | 3.45 | 3.62 ---------------------------------------- {269}