PHYSICAL PROPERTIES OF WATER, ETC. 69 



steam might be condensed isothermally to supersaturation without condensing. Hence 

 he 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. Thomson's was developed, in 

 connection with a remarkable speculation of "W. Thomson, 1 on the pressure of vapour as 

 depending on the curvature 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 be 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 pres- 

 sures, either the vapour condenses suddenly into liquid, or the liquid suddenly bursts 

 into vapour. 



" If this were the case, no nucleus would be absolutely requisite for the formation 

 either of liquid from vapour or of vapour from liquid. All that would be required, in 

 either case, would be the proper increase or diminution of pressure ; — temperature being 

 kept unaltered. 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 see, on this theory, how it can be explosively 

 taken in on the sudden passage from liquid to vapour. 



" Aitken's experiments tend to show, what J. Thomson only speculatively announced, 

 that possibly vapour may not be condensed (in the absence of a nucleus), when com- 

 pressed isothermally, even at ranges far beyond the maximum of pressure indicated in 

 Thomson's figures. Hence it would appear that the range of instability is much less 

 than that given by Thomson's figures, and may (perhaps) be looked on as a vanishing 

 quantity ; the corresponding part of the isothermal being a finite line parallel to the 

 axis of pressures, corresponding to the sudden absorption or giving out of latent 

 heat." 



1 Proc. Roy Soc. Edin., vol. vii. p. 63, 1870. 



