467 
1909 - 10 .] Isothermal Change of State. 
true conditions which subsist in the passage, if it be presumed possible, of a 
constantly homogeneous substance from one molecular state to another. 
Here the word homogeneous is to be understood in the only sense in which 
it can be employed wi th reference to a molecularly constituted medium. If 
there be even a single effective nucleus present, and even if that nucleus be 
a portion of the same substance in a different state of average molecular 
aggregation from the remainder, the complex is, in this sense, non- 
homogeneous. 
James Thomson’s suggestion that the true surface, in the liquid- vapour 
region, passes, in continuation of its normal liquid portion, to the smaller 
pressure side of the cylindrical surface, and passes, in continuation of its 
normal vapour portion, to the greater pressure side of the cylindrical surface, 
both extensions finally curving right round and joining each other along a 
line of inflection, secured geometrical continuity, but, as Maxwell remarked, 
did not secure physical continuity, for there is necessarily physical 
instability in the intermediate region where pressure and volume increase 
or decrease together. Van der Waal’s and other equations of state give a 
theoretical deduction of Thomson’s form of representation. 
The existence of physical instability would, on the molecular theory, be 
representable as the result of molecular repulsion increasing with average 
molecular distance ; and the instability could be prevented, by external 
control, from manifesting itself in an explosion. The substance might be 
enclosed in a vessel provided with a screw plunger working with sufficient 
frictional resistance to prevent its rotation under the largest axial pressure 
supplied by the substance in the process of transformation. Just sufficient 
external force applied to the plunger would be followed by outward motion 
of the plunger, which in consequence would be removed from the required 
external action, so that its motion would cease as instantaneously as might 
be desired. If the external work performed by the substance during 
the expansion in this region were greater than the loss of internal 
energy, latent heat would have to be supplied. Thus, apart from difficulties 
arising from possible nucleation, the whole course of one of Thomson’s 
isothermals might be traced in such a case. 
3. Thomson’s considerations are as applicable to the passage of a sub- 
stance from the solid state to the liquid state, and from the solid state to 
the state of vapour, as to the passage between the liquid and the vapour 
states. Hence, below the triple-point — at which normally (i.e. with nuclea- 
tion) all three states coexist stably — we have to recognise the existence of 
three regions of instability on any one isothermal. 
As the point characteristic of the state of the substance moves along an 
