| | | | -- | -- | 7.5 | 10 | 17.5 | | | | 51 | 9.5 | -- | 3.5 | 64 | | | | | | | | 40.5 | 13 | -- | 3.5 | 57 | | | | | | | | 18 | 23 | -- | 3 | 44 ------------------------------------------------------------------------- From the aspect of these diagrams the conditions under which the salts can coexist can be read at a glance. Thus, {319} for example, Fig. 124 shows that at 0° Glauber's salt and potassium chloride can exist together with solution; namely, in contact with solutions having the composition X--XI. This temperature must therefore be below the transition point of this salt-pair (p. 314). On raising the temperature to 4.4°, it is found that the curve VIII.--XI. moves so that the point XI. coincides with point X. At this point, therefore, there will be _four_ concurrent fields, viz. Glauber's salt, potassium chloride, glaserite, and sodium chloride. But these four salts can coexist with solution only at the transition point; so that 4.4° is the transition temperature of the salt-pair: Glauber's salt--potassium chloride. At higher temperatures the line VIII.--XI. moves still further to the left, so that the field for Glauber's salt becomes entirely separated from the field for potassium chloride. This shows that at temperatures above the transition point the salt-pair Glauber's salt--potassium chloride cannot coexist in presence of solution. [Illustration: FIG. 125.] If it is only desired to indicate the mutual relationships of the different components and the conditions for their coexistence (_paragenesis_), a simpler diagram than Fig. 124 can be employed. Thus if the boundary curves of Fig. 124 are so drawn that they cut one another at right angles, a figure such as Fig. 125 is obtained, the Roman numerals here corresponding with those in Fig. 124. Ammonia-Soda Process.--One of the most important applications of the Phase Rule to systems of four components with reciprocal salt-pairs has recently been made by Fedotieff[392] in his investigations of the conditions for the formation of sodium carbonate by the so-called ammonia-soda (Solvay) {320} process.[393] This process consists, as is well known, in passing carbon dioxide through a solution of common salt saturated with ammonia. Whatever differences of detail there may be in the process as carried out in different manufactories, the reaction which forms the basis of the process is that represented by the equation NaCl + NH_{4}HCO_{3} = NaHCO_{3} + NH_{4}Cl