of Edinburgh, Session 1834 - 85 . 
79 
Thomson’s work.] He pointed out that there were numerous ex- 
periments proving that water could be heated, under certain condi- 
tions, far above its boiling point without evaporating ; and that, 
probably, steam might be condensed isothermally to supersaturation 
without condensing. Hence be was led to suggest an isothermal of 
continued curvature, instead of the broken line given by Andrews, 
as representing the continuous passage of a fluid from the state of 
vapour to that of liquid ; the whole mass being supposed to be, at 
each stage of the process, in the same molecular state. 
In Clerk-Maxwell’s Treatise on Heat, this idea of J. Thom- 
son’s was developed, in connection with a remarkable speculation of 
W. Thomson,* on the pressure of vapour as depending on the cur- 
vature of the liquid surface in contact with it. This completely 
accounts for the deposition of vapour when a proper nucleus is 
present. Maxwell showed that it could also account for the 
“ singing ” of a kettle, and for the growth of the larger drops in a 
cloud at the expense of the smaller ones. 
The main objection to J. Thomson’s suggested isothermal curve 
of transition is that, as Maxwell points out, it contains a region in 
which pressure and volume increase or diminish simultaneously. 
This necessarily involves instability ; inasmuch as, for definite values 
of pressure at constant temperature within a certain range in which 
vapour and liquid can he in equilibrium, Thomson’s hypothesis leads 
to three different values of volume: two of which are stable ; but the 
intermediate one essentially unstable. According to Maxwell, the 
extremities of this triple region correspond to pressures, at which, 
regarded from the view of steady increase or diminution of pressures, 
either the vapour condenses suddenly into liquid, or the liquid 
suddenly bursts into vapour. 
If this were the case, no nucleus would he absolutely requisite for 
the formation either of liquid from vapour or of vapour from liquid. 
All that would be required, in either case, would he the proper 
increase or diminution of pressure; — temperature being kept un- 
altered. The latent heat of vapour, which we know to become less 
as the critical point is gradually arrived at, would thus be given off 
in the explosive passage from vapour to liquid. It is difficult to 
Proc. Roy. Soc. Eclin., 1870 . 
