From the Phase Rule, however, we learn that, in order that at a given temperature the pressure of a two-component system {88} may be constant, there must be three phases present. Strictly, therefore, we can speak only of the vapour pressure of a _system_; and since, in the cases under discussion, the hydrates dissociate into a solid and a vapour, any statement as to the vapour pressure of a hydrate has a definite meaning _only when the second solid phase produced by the dissociation is given_. The everyday custom of speaking of the vapour pressure of a hydrated salt acquires a meaning only through the assumption, tacitly made, that the second solid phase, or the solid produced by the dehydration of the hydrate, is the _next lower_ hydrate, where more hydrates than one exist. That a hydrate always dissociates in such a way that the next lower hydrate is formed is, however, by no means certain; indeed, cases have been met with where apparently the anhydrous salt, and not the lower hydrate (the existence of which was possible), was produced by the dissociation of the higher hydrate.[156] That a salt hydrate can exhibit different vapour pressures according to the solid product of dissociation, can not only be proved theoretically, but it has also been shown experimentally to be a fact. Thus CaCl_{2},6H_{2}O can dissociate into water vapour and either of two lower hydrates, each containing four molecules of water of crystallization, and designated respectively as CaCl_{2},4H_{2}O[alpha], and CaCl_{2},4H_{2}O[beta]. Roozeboom[157] has shown that the vapour pressure which is obtained differs according to which of these two hydrates is formed, as can be seen from the following figures:-- -------------+---------------------------------------------------------- | Pressure of System. Temperature. +-----------------------------+---------------------------- | CaCl_{2},6H_{2}O; CaCl_{2}, | CaCl_{2},6H_{2}O; CaCl_{2}, | 4H_{2}O[alpha]; vapour. | 4H_{2}O[beta]; vapour. -------------+-----------------------------+---------------------------- -15° | 0.027 cm. | 0.022 cm. 0 | 0.092 " | 0.076 " +10 | 0.192 " | 0.162 " 20 | 0.378 " | 0.315 " 25 | 0.508 " | 0.432 " 29.2 | -- | 0.567 " 29.8 | 0.680 " | -- -------------+-----------------------------+--------------------------- {89} By reason of the non-recognition of the importance of the solid dissociation product for the definition of the dissociation pressure of a salt hydrate, many of the older determinations lose much of their value.