If a _dilute_ aqueous saline solution be taken at ordinary
temperatures, and then slowly cooled to some point below zero on the
Centigrade scale, the following series of changes will in general be
observed: On reaching a point below zero, the position of which is
dependent upon the nature of the salt and the amount of dilution, it
will be found that ice is formed; this will float upon the surface of
the solution, and may be readily removed. If the ice so removed be
afterward pressed, or carefully drained, it will be found to consist
of nearly pure water, the liquid draining away being a strong saline
solution which had become mechanically entangled among the crystals of
ice during solidification. If we further cool the brine which remains,
we notice a tolerably uniform fall of temperature with accompanying
formation of ice. But at length a point is reached at which the
temperature ceases to fall until the whole of the remaining
mother-liquor has solidified, with the production of a compound called
a cryohydrate,[3] which possesses physical properties different from
those of either the ice or the salt from which it is formed.
[Footnote 3: Guthrie, _Phil. Mag._, 4th Series, xlix., pp. 1,
206, 266; 5th Series, i., pp. 49, 354, 446, vi., p. 35.]
If, on the other hand, we commence with a _saturated_ saline solution,
in general it is noticed on cooling the liquid a separation of salt
ensues, which salt sinks to the bottom of the mass, and may be
removed. The salt so separating may be either anhydrous or a "hydrate"
of greater concentration than the mother-liquor. So long as this
separation proceeds the temperature falls, but at length a point is
reached at which the thermometer remains stationary until the whole is
solidified, with the production of a cryohydrate. This temperature of
solidification is the same whether we start with a dilute or a
saturated solution, and the composition of the cryohydrate is found to
be constant. The temperature of production of the cryohydrate is
identical with the lowest temperature which can be produced on
employing a mixture of ice and the salt as a freezing mixture or
cryogen.
It will be readily seen that in the formation of a cryohydrate we have
an example of eutexia, since the constituents are present in such
proportion as to give to the resultant compound body a minimum
temperature of liquefaction.