It must be noted, however, that the four salts formed by two reciprocal salt-pairs do not constitute a system of four, but only of _three_ components. This will be understood if it is recalled that only so many constituents are taken as components as are necessary to _express_ the composition of all the phases present (p. 12). It will be seen, now, that the composition of each of the four salts which can be present together can be expressed in terms of three of them. Thus, for example, in the case of NH_{4}Cl, NaNO_{3}, NH_{4}NO_{3}, NaCl, we can express the composition of NH_{4}Cl by NH_{4}NO_{3} + NaCl - NaNO_{3}; or of NaNO_{3} by NH_{4}NO_{3} + NaCl - NH_{4}Cl. In all these cases it will be seen that negative quantities of one of the components must be employed; but that we have seen to be quite permissible (p. 12). The number of components is, therefore, three; but any three of the four salts can be chosen. Since, then, two reciprocal salt-pairs constitute only three {313} components or independently variable constituents, another component is necessary in order to obtain a four-component system. As such, we shall choose water. Transition Point.--In the case of the formation of double salts from two single salts, we saw that there was a point--the _quintuple point_--at which five phases could coexist. This point we also saw to be a transition point, on one side of which the double salt, on the other side the two single salts in contact with solution, were found to be the stable system. A similar behaviour is found in the case of reciprocal salt-pairs. The four-component system, two reciprocal salt-pairs and water, can give rise to an invariant system in which the six phases, four salts, solution, vapour, can coexist; the temperature at which this is possible constitutes a _sextuple point_. Now, this sextuple point is also a transition point, on the one side of which the one salt-pair, on the other side the reciprocal salt-pair, is stable in contact with solution. The sextuple point is the point of intersection of the curves of six univariant systems, viz. four solubility curves with three solid phases each, a vapour-pressure curve for the system: two reciprocal salt-pairs--vapour; and a transition curve for the condensed system: two reciprocal salt-pairs--solution. If we omit the vapour phase and work under atmospheric pressure (in open vessels), we find that the transition point is the point of intersection of four solubility curves.