1871.] Abrupt Change at Boiling or Condensing fyc. 7 



admitting speedily of dry steam nearly at its condensing temperature for 

 its pressure (or, to use a common expression, nearly saturated) into a 

 vessel with a piston or plunger, all kept hotter than the steam, and then 

 allowing the steam to expand till by its expansion it would be cooled below 

 its condensing-point for its pressure ; and yet I would suppose that if this 

 were done with very careful precautions the steam might not condense, on 

 account of the cooled steam being surrounded entirely with a thin film of 

 superheated steam close to the superheated containing vessel. The fact 

 of its not condensing might perhaps best be ascertained by observations on 

 its volume and pressure. Such an experiment as that sketched out here 

 would not be easily made, and unless it were conducted with very great 

 precautions, there could be no reasonable expectation of success in its at- 

 tempt ; and perhaps it might not be possible so completely to avoid the 

 presence of dust or other dense particles in the steam as to make it prove 

 successful. I mention it, however, as appearing to be founded on correct 

 principles, and as tending to suggest desirable courses for experimenral re- 

 searches. The overhanging part of the curve from e to / seems to repre- 

 sent a state in which there would be some kind of unstable equilibrium ; 

 and so, although the curve there appears to have some important theore- 

 tical significance, yet the states represented by its various points would be 

 unattainable throughout any ordinary mass of the fluid. It seems to re- 

 present conditions of coexistent temperature, pressure, and volume in 

 which, if all parts of a mass of fluid were placed, it would be in equili- 

 brium, but out of which it would be led to rush, partly into the rarer state 

 of gas, and partly into the denser state of liquid, by the slightest inequality 

 of temperature or of density in any part relatively to other parts. I might 

 proceed to state, in support of these views, several considerations founded 

 on the ordinary statical theory of capillary or superficial phenomena of 

 liquids, which is dependent on the supposition of an attraction acting 

 very intensely for very small distances, and causing intense pressure in 

 liquids over and above the pressure applied by the containing vessel and 

 measurable by any pressure-gauge. That statical theory has fitted re- 

 markably well to many observed phenomena, and has sometimes even 

 led to the forecasting of new results in advance of experiment. Hence, 

 although dynamic or kinetic theories of the constitution and pressure of 

 fluids now seem likely to supersede any statical theory, yet phenomena 

 may still be discussed according to the principles of the statical theory ; 



stances with other substances, experience a difficulty of making a beginning of their 

 change of state, whether from liquid to solid, or from liquid to gaseous, or probably also 

 from solid to liquid," was proposed by me, and, so far as I am aware, was first announced 

 in a paper by me in the Proceedings of the Boyal Society for November 24, 1859 (vol. x. 

 p. 158), and in a paper submitted to the British Association in the same year. 



In the present paper, at the place to which this note is annexed, I adduce the like 

 further supposition that a difficulty of making a beginning of change of state from 

 gaseous to liquid may also probably exist. 



