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which represents the case that the two components are not subject 
to transformation. 
For this another new representation may be given which consi- 
ders the matter from a more general point of view. 
Taking p, ¢ and & as coordinates, a surface may be constructed 
which shall represent the equilibrium between the two kinds of 
molecules in a homogeneous phase, vapour or liquid. 
Ac. Conc. Par. Ae. Cone. Pa. 
Fig. 2. Fig. 3. 
The general form of such a surface of equilibrium for the system 
acet-paraldehyde may be readily deduced from analogy with other 
known equilibria in the gaseous condition, if one considers that 
paraldehyde requires heat to pass into acetaldehyde and may be 
reobtained from the same by compression. 
The general course of the equilibrium line at a constant pressure 
is indicated in fig. 2, that at constant temperature in fig. 3. If we 
now imagine that on the different points of the ¢, v-line in a hori- 
zontal plane, p,-lines are erected in vertical planes, we obtain a 
Pp, t, & surface of a very peculiar shape which gives the equilibrium 
relation between acetaldehyde and paraldehyde for every temperature 
and pressure. 
The course may be theoretically calculated for the vapour if the 
pressure is not too large. With greater pressures and for the liquid 
state this becomes a difficult matter but the general course remains 
fairly certain. We might therefore, imagine this equilibrium surface 
first of all at temperatures higher than those of the critical curve 
LM. Here, the surface would for some time extend itself undisturbed 
both vertically and horizontally. At lower temperatures, the surface, 
on account of its form, must necessarily meet first of all the 
surface for liquid-vapour; according to the investigation this takes 
place in the point P. From here to lower temperatures, the 
