Summary.--With regard to double salts we have learned that their formation from and their decomposition into the single salts, is connected with a definite temperature, the _transition temperature_. At this transition temperature two vapour pressure curves cut, viz. a curve of dehydration of a mixture of the single salts and the solubility curve of the double salt; or the dehydration curve of the double salt and the solubility curve of the mixed single salts. The solubility curves, also, of these two systems intersect at the transition point, but although the formation of the double salt commences at the transition point, complete stability in contact with water may not be attained till some temperature above (or below) that point. _Only when the temperature is beyond the transition interval, will a double salt dissolve in water without decomposition (_e.g._ the alums)._ * * * * * {272} CHAPTER XVI ISOTHERMAL CURVES AND THE SPACE MODEL In the preceding chapter we considered the changes in the solubility of double salts and of mixtures of their constituent salts with the temperature; noting, more especially, the relationships between the two systems at the transition point. It is now proposed to conclude the study of the three-component systems by discussing very briefly the solubility relations at constant temperature, or the isothermal solubility curves. In this way fresh light will be thrown on the change in the solubility of one component by the addition of another component, and also on the conditions of formation and stable existence of double salts in solution. With the help of these isothermal curves, also, the phenomena of crystallization at constant temperature--phenomena which have not only a scientific interest but also an important bearing on the industrial preparation of double salts--will be more clearly understood.[354] A brief description will also be given of the method of representing the variation of the concentration of the two salts in the solution with the temperature.