VAPOUR PRESSURE OF SOL UTIONS 43

form of a projection (Fig. 13). TakeDAF, Fig. n, showing
the composition for horizontal plane ; then E^B^ the pres-
sure diagram, Fig. 4, may be reproduced on the vertical
plane. D then lies on the axis OT ; A is given by the pro-
jections AJ and A. The pressure diagram EIDIBI refers, as
Fig. 4 did, on the left of EIDI to ice, right of that to water.
r^ corresponds to the pressure of pure water at the freezing
point, and consequently lies above AXM, which refers to
the saturated solution ; DJ also lies in the continuation of
the curve o AJ , since that, like AIDI , gives the pressure of
ice; from Dt to the right lies the vapour pressure curve -
for water. Further, the boundary between water and ice,
T>i&l9 may be drawn from B1 upwards, and from Al the line
A1a1 of cryohydric pressure, which is given on the hori-
zontal plane by a line corresponding to the composition of
the cryohydric solutions for different pressures. The new
areas, given only by the projection, relate to conditions not
previously taken into account :

FMADB,, unsaturated solution and vapour, bounded by
AMF for the saturated solution, DBa for water, and AD for
the freezing point

GIADEI, unsaturated solution and ice, bounded by atA for the
cryohydric state, EjD for water, and AD for the melting point.

(j3) Pressure of the unsaturated solution. The theorems
and results developed so far refer to the state of so-called
saturation, at least so far as thej' have been followed out
thermo- dynamically ; only in the graphical representation
finally adopted was a place found for the unsaturated con-
dition. This place may be exactly defined by a series of
measurements with successively greater concentrations. Each
pressure curve so found must correspond with an equation

dT
in which the value of q must be considered separately in
each case, arid stands for the heat developed when 18 kilo-
grams of vapour, after condensation — with evolution of