If an unsaturated solution of the two salts in equimolecular amounts is evaporated, potassium chloride will first be deposited, because the plane bisecting the right angle formed by the X and Y axes cuts the area for that salt. Deposition of potassium chloride will lead to a relative increase in the concentration of magnesium chloride in the solution; and on continued evaporation a point (on the curve EM) will be reached at which carnallite will separate out. So long as the two solid phases are present, the composition of the solution must remain unchanged. Since the separation of carnallite causes a decrease in the relative concentration of the potassium chloride in the solution, the portion of this salt which was deposited at the commencement must _redissolve_, and carnallite will be left on evaporating to dryness. (_Incongruently saturated solution._) Although carnallite is decomposed by pure water, it will be possible to crystallize it from a solution having a composition represented by any point in the carnallite area. Since during the separation of the double salt the relative amount of magnesium chloride increases, it is most advantageous to {290} commence with a solution the composition of which is represented by a point lying just above the curve EM (cf. p. 281). From the above description of the behaviour of carnallite in solution, the processes usually employed for obtaining potassium chloride will be readily intelligible.[367] Ferric Chloride--Hydrogen Chloride--Water.--In the case of another system of three components which we shall now describe, the relationships are considerably more complicated than in those already discussed. They deserve discussion, however, on account of the fact that they exhibit a number of new phenomena. In the system formed by the three components, ferric chloride, hydrogen chloride, and water, not only can various compounds of ferric chloride and water (p. 152), and of hydrogen chloride and water be formed, each of which possesses a definite melting point, but various ternary compounds are also known. Thus we have the following solid phases:-- 2FeCl_{3},12H_{2}O HCl,3H_{2}O 2FeCl_{3},2HCl,12H_{2}O 2FeCl_{3},7H_{2}O HCl,2H_{2}O 2FeCl_{3},2HCl,8H_{2}O 2FeCl_{3},5H_{2}O HCl,H_{2}O 2FeCl_{3},2HCl,4H_{2}O 2FeCl_{3},4H_{2}O FeCl_{3} From this it will be readily understood that the complete study of the conditions of temperature and concentration under which solutions can exist, either with one solid phase or with two or three solid phases, are exceedingly complicated; and, as a matter of fact, only a few of the possible equilibria have been investigated. We shall attempt here only a brief description of the most important of these.[368]