are equal; at this temperature the four phases, Na_{2}SO_{4},10H_{2}O; Na_{2}SO_{4}; solution; vapour, can coexist. From this it is evident that when sodium sulphate decahydrate is heated to 32.6°, the two new phases anhydrous salt and solution will be formed (suspended transformation being supposed excluded), and the hydrate will appear to undergo _partial fusion_; and during the process of "melting" the vapour pressure and temperature will remain constant.[213] This is, however, not a true but a so-called _incongruent_ melting point; for the composition of the liquid phase is not the same as that of the solid. As has already been pointed out (p. 137), we are dealing here with the _transition point_ of the decahydrate and anhydrous salt, _i.e._ with the reaction Na_{2}SO_{4},10H_{2}O <--> Na_{2}SO_{4} + 10H_{2}O. Since at the point of partial fusion of the decahydrate four {140} phases can coexist, the point is a quadruple point in a two-component system, and the system at this point is therefore invariant. The temperature of this point is therefore perfectly definite, and on this account the proposal has been made to adopt this as a fixed point in thermometry.[214] The temperature is, of course, practically the same as that at which the two solubility curves intersect (p. 112). If, however, the vapour phase disappears, the system becomes univariant, and the equilibrium temperature undergoes change with change of pressure. The transition curve has been determined by Tammann,[215] and shown to pass through a point of maximum temperature. [Illustration: FIG. 34.] The vapour pressure of the different systems of sodium sulphate and water can best be studied with the help of the diagram in Fig. 34.[216] The curve ABCD represents the vapour-pressure curve of the saturated solution of anhydrous sodium sulphate. GC is the pressure curve of decahydrate + anhydrous salt, which, as we have seen, cuts the curve ABCD at the transition temperature, 32.6°. Since at this point the solution is saturated with respect to both the anhydrous salt and the decahydrate, the vapour-pressure curve of the saturated solution of the latter must also pass through the point C.[217] As at temperatures below this point the solubility of the decahydrate is less than that of the anhydrous salt, the vapour pressure of the solution will, in accordance with Babo's law (p. 126), be higher than that of the solution of the anhydrous salt; which was also found experimentally to be the case (curve HC). {141}