The outline of this figure represents four ternary solutions in which the component salts have a common acid or basic constituent; viz. sodium chloride--sodium sulphate, sodium sulphate--potassium sulphate, potassium sulphate--potassium chloride, potassium chloride--sodium chloride. These four sets of curves are therefore similar to those discussed in the previous chapter. In the case of sodium and potassium sulphate, a double salt, _glaserite_ [K_{3}Na(SO_{4})_{2}] is formed. Whether glaserite is really a definite compound or not is still a matter of doubt, since isomorphic mixtures of Na_{2}SO_{4} and K_{2}SO_{4} have been obtained. According to van't Hoff and Barscholl,[389] glaserite is an isomorphous mixture; but Gossner[390] considers it to be a definite compound having the formula K_{3}Na(SO_{4})_{2}. Points VIII. and IX. represent solutions saturated with respect to glaserite and sodium sulphate, and glaserite and potassium sulphate respectively. The lines which pass inwards from these boundary curves represent solutions containing three salts, but in contact with only two solid phases; and the points where three lines meet, or where three fields meet, represent solutions in equilibrium with three solid phases; with the phases, namely, belonging to the three concurrent fields. If it is desired to represent a solution containing the salts say in the proportions, 51Na_{2}Cl_{2}, 9.5K_{2}Cl_{2}, 3.5K_{2}SO_{4}, the difficulty is met with that two of the salts, sodium chloride and potassium sulphate, lie on opposite axes. To overcome this difficulty the difference 51 - 3.5 = 47.5 is taken and measured off along the sodium chloride axis; and the solution is therefore represented by the point 47.5Na_{2}Cl_{2}, 9.5K_{2}Cl_{2}. In order, therefore, to find the amount of potassium sulphate present {318} from such a diagram, it is necessary to know the total number of salt molecules in the solution. When this is known, it is only necessary to subtract from it the sum of the molecules of sodium and potassium chloride, and the result is equal to twice the number of potassium sulphate molecules. Thus, in the above example, the total number of salt molecules is 64. The number of molecules of sodium and potassium chloride is 57; 64 - 57 = 7, and therefore the number of potassium sulphate molecules is 3.5. Another method of representation employed is to indicate the amounts of only two of the salts in a plane diagram, and to measure off the total number of molecules along a vertical axis. In this way a solid model is obtained. The numerical data from which Fig. 124 was constructed are contained in the following table, which gives the composition of the different solutions at 0°:--[391]