70 THE VOYAGE OF H.M.S. CHALLENGER. 



ON EVAPORATION AND CONDENSATION. 



" While I was communicating my Note on the Necessity for a Condensation Nucleus 

 at the last meeting of the Society, an idea occurred to me which germinated (on my 

 way home) to such an extent that I sent it off by letter to Professor J. Thomson that 

 same night. 



" J. Thomson's idea, which I had been discussing, was to preserve, if possible, 

 physical (as well as geometrical) continuity in the isothermal of the liquid-vapour 

 state, by keeping the whole mass of fluid in one state throughout. He secured 

 geometrical, but not physical, continuity. For, as Clerk-Maxwell showed, one part of 

 his curve makes pressure and volume increase simultaneously, a condition essentially 

 unstable. The idea which occurred to me was, while preserving geometrical continuity, 

 to get rid of the region of physical instability, not (as I had suggested in my former 

 Note) by retaining Thomson's proposed finite maximum and minimum of pressure in 

 the isothermal, while bringing them infinitely close together so far as volume is con- 

 cerned, and thus restricting the unstable part of the isothermal to a finite line parallel 

 to the pressure axis ; but, by making both the maximum and minimum infinite. 

 Geometrical continuity, of course, exists across an asymptote parallel to the axis of 

 pressures ; so that, from this point of view, there is nothing to object to. On the 

 other hand, there is essentially physical discontinuity, in the form of an impassable 

 barrier between the vaporous and liquid states, so long at least as the substance is 

 considered as homogeneous throughout. 



" It appeared to me that here lies the true solution of the difficulty. As we are 

 dealing with a fluid mass essentially homogeneous throughout, it is clear that we are 

 not concerned with cases in which there is a molecular surface-film. 



" Suppose, then, a fluid mass, somehow maintained at a constant temperature 

 (lower than its critical point), and so extensive that its boundaries may be regarded 

 as everywhere infinitely distant, what will be the form of its isothermal in terms of 

 pressure and volume ? 



" Two prominent experimental facts help us to an answer. 



" First. We know that the interior of a mass of liquid mercury can be subjected to 

 hydrostatic tension of considerable amount without rupture. The isothermal must, in 

 this case, cross the line of volumes ; and the limit of the tension would, in ordinary 

 language, be called the cohesion of the liquid. I am not aware that this result has 

 been obtained with water free from air ; but possibly the experiment has not been 

 satisfactorily made. The common experiment in which a rough measure is obtained 

 of the force necessary to tear a glass plate from the surface of water is vitiated by the 

 instability of the concave molecular film formed. 



