Just as in the case of three-component systems we saw that the presence of one of the single salts along with the double salt was necessary in order to give a univariant system, so in the four-component systems the presence of a third salt is necessary as solid phase along with one of the salt-pairs. In the case of the reciprocal salt-pairs mentioned above, the transition point would be the point of intersection of the solubility curves of the systems with the following groups of salts as solid phases: Below the transition point: NH_{4}Cl + NaNO_{3} + NaCl; NH_{4}Cl + NaNO_{3} + NH_{4}NO_{3}; above the transition point: NaCl + NH_{4}NO_{3} + NaNO_{3}; NaCl + NH_{4}NO_{3} + NH_{4}Cl. From this we see that the two salts NH_{4}Cl and NaNO_{3} would be able to exist together with solution below the transition point, but not above it. This transition point has not been determined. {314} Formation of Double Salts.--In all cases of four-component systems so far studied, the transition points have not been points at which one salt-pair passed into its reciprocal, but at which a double salt was formed. Thus, at 4.4° Glauber's salt and potassium chloride form glaserite and sodium chloride, according to the equation 2Na_{2}SO_{4},10H_{2}O + 3KCl = K_{3}Na(SO_{4})_{2} + 3NaCl + 20H_{2}O Above the transition point, therefore, there would be K_{3}Na(SO_{4})_{2}, NaCl and KCl; and it may be considered that at a higher temperature the double salt would interact with the potassium chloride according to the equation K_{3}Na(SO_{4})_{2} + KCl = 2K_{2}SO_{4} + NaCl thus giving the reciprocal of the original salt-pair. This point has, however, not been experimentally realized.[384] Transition Interval.--A double salt, we learned (p. 277), when brought in contact with water at the transition point undergoes partial decomposition with separation of one of the constituent salts; and only after a certain range of temperature (transition interval) has been passed, can a pure saturated solution be obtained. A similar behaviour is also found in the case of reciprocal salt-pairs. If one of the salt-pairs is brought in contact with water at the transition point, interaction will occur and one of the salts of the reciprocal salt-pair will be deposited; and this will be the case throughout a certain range of temperature, after which it will be possible to prepare a solution saturated only for the one salt-pair. In the case of ammonium chloride and sodium nitrate the lower limit of the transition interval is 5.5°, so that above this temperature and up to that of the transition point (unknown), ammonium chloride and sodium nitrate in contact with water would give rise to a third salt by double decomposition, in this case to sodium chloride.[385] {315}