. Stoichiometry . ve, as in the case of a pure substance,but by a surface enclosed by a border curve (Kuenen). The generalform of the vapour-pressure surface is mdicated by the curves VRPLin Figs. 71, 72, and 74. It may safely be assumed that the relations between pressure,temperature, and composition are simplest in the case of mixturesof substances which are chemically closely related ; but, unfortunately,the only determinations which have so far been made with such 204 STOICHIOMETRY substances ^ are not sufficiently complete, as regards range of com-position, to allow of definite conclusion
. Stoichiometry . ve, as in the case of a pure substance,but by a surface enclosed by a border curve (Kuenen). The generalform of the vapour-pressure surface is mdicated by the curves VRPLin Figs. 71, 72, and 74. It may safely be assumed that the relations between pressure,temperature, and composition are simplest in the case of mixturesof substances which are chemically closely related ; but, unfortunately,the only determinations which have so far been made with such 204 STOICHIOMETRY substances ^ are not sufficiently complete, as regards range of com-position, to allow of definite conclusions being drawn regarding therelation between composition and critical pressure or critical tem-perature. Several pairs of infinitely miscible liquids which do not formmixtures of maximum or minimum pressure have, however, beencarefully examined by Kuenen and by Caubet, and the results ob-tained by the latter observer with mixtures of carbon dioxide andsulphur dioxide are shown in Fig. 71. The vapour-pressure curves. 5 Volume Fig, 70. of the two pure substances terminate in their critical points, C^ andC2 respectively. The vapour-pressure areas enclosed by the bordercurves (VRPL) for the mixtures investigated lie between these curvesand a third curve, called by Kuenen i\\c J>lai/ point curve, drawnbetween C^ and Co is tangential to the border curves. The point ofcontact P between a border curve and the plait point curve istermed by Kuenen the plait point temperature, and by Caubetthe critical temperature. The point R at which the border curvehas a tangent parallel to the/-axis is termed by Kuenen the critical Kuenen, Ethane and Butane, Proc. Roy. Soc. Ediii., vol. 21. MIX TURES 265 point of contact; it gives the maximum temperature on the bordercurve. At all temperatures below the plait point /p, vaporization or con-densation at constant temperature takes place in the ordinarymanner ; starting with saturated vapour and compressing the mixture,condensation takes place, and the
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