When, as is the case in industrial practice, we are dealing with solutions which are saturated not for two salts but only for sodium bicarbonate, it is evident that we have then to do with solutions the composition of which is represented by points in the area P_{1}P_{2}I,IV. Since in the commercial manufacture, the aim must be to obtain as complete a utilization of the materials as possible, the solutions employed industrially must lie in the neighbourhood of the curves P_{2}P_{1}IV, as is indicated by the shaded portion in Fig. 127. The best results, from the manufacturer's standpoint, will be obtained, as already stated, when the composition of the solutions approaches that given by a point on the curve P_{2}P_{1}. Considered from the chemical standpoint, the results of the experiments lead to the conclusion that the Solvay process, _i.e._ passage of carbon dioxide through a solution of sodium chloride saturated with ammonia, is not so good as the newer method of Schlösing, which consists in bringing together sodium chloride and ammonium bicarbonate with water.[395] {326} Preparation of Barium Nitrite.--Mention may also be made here of the preparation of barium nitrite by double decomposition of barium chloride and sodium nitrite.[396] The reaction with which we are dealing here is represented by the equation BaCl_{2} + 2NaNO_{2} = 2NaCl + Ba(NO_{2})_{2} It was found that at the ordinary temperature NaCl and Ba(NO_{2})_{2} form the stable salt-pair. If, therefore, barium chloride and sodium nitrite are brought together with an amount of water insufficient for complete solution, transformation to the stable salt-pair occurs, and sodium chloride and barium nitrite are deposited. When, however, a stable salt-pair is in its transition interval (p. 315), a third salt--in this case barium chloride--will be deposited, as we have already learned. On bringing barium chloride and sodium nitrite together with water, therefore, three solid phases are obtained, viz. BaCl_{2}, NaCl, Ba(NO_{2})_{2}. These three phases, together with solution and vapour, constitute a univariant system, so that at each temperature the composition of the solution must be constant.