Witt and Ludwig found that the presence of solid barium chloride can be prevented by adding an excess of sodium nitrite, as can be readily foreseen from what has been said. Since the solution in presence of the three solid phases must have a definite composition at a definite temperature, the addition of sodium nitrite to the solution must have, as its consequence, the solution of an equivalent amount of barium chloride, and the deposition of an equivalent amount of sodium chloride and barium nitrite. By sufficient addition of sodium nitrite, the complete disappearance of the solid barium chloride can be effected, and there will remain only the stable salt-pair sodium chloride and barium nitrite. As was pointed out by Meyerhoffer, however, the disappearance of the barium chloride is effected, not by a change in the {327} composition of the solution, but by the necessity for the composition of the solution remaining constant. [Illustration: FIG. 128.] Barium Carbonate and Potassium Sulphate.--As has been found by Meyerhoffer,[397] these two salts form the stable pair, not only at the ordinary temperature, but also at the melting point. For the ordinary temperatures this was proved in the following manner: A solution with the solid phases K_{2}SO_{4} and K_{2}CO_{3}.2H_{2}O in excess can only coexist in contact either with BaCO_{3} or with BaSO_{4}, since, evidently, in one of the two groups the stable system must be present. Two solutions were prepared, each with excess of K_{2}SO_{4} + K_{2}CO_{3}.2H_{2}O, {328} and to one was added BaCO_{3} and to the other BaSO_{4}. After stirring for a few days, the barium sulphate was completely transformed to BaCO_{3}, whereas the barium carbonate remained unchanged. Consequently, BaCO_{3} + K_{2}SO_{4} + K_{2}CO_{3}.2H_{2}O is stable, and, therefore, so also is BaCO_{3} + K_{2}SO_{4}. That BaCO_{3} + K_{2}SO_{4} is the stable pair also at the melting point was proved by a special analytical method which allows of the detection of K_{2}CO_{3} in a mixture of the four solid salts. This analysis showed that a mixture of BaCO_{3} + K_{2}SO_{4}, after being fused and allowed to solidify, contains only small amounts of K_{2}CO_{3}; and this is due entirely to the fact that BaCO_{3} + K_{2}SO_{4} on fusion deposits a little BaSO_{4}, thereby giving rise at the same time to the separation of an equivalent amount of K_{2}CO_{3}.