2 



Prof. J. Thomson on the 



[Nov. 16, 



gaseous, and the other as being an ordinary liquid state of the same matter; 

 and the ordinary transition from the one to the other we would designate 

 by the terms boiling or condensing, or occasionally by other terms nearly 

 equivalent, such as evaporation, gasification, liquefaction from the gaseous 

 state, &c. Cases of gasification from liquids or of condensation from 

 gases, when any chemical alteration accompanies the abrupt change of 

 density, are not among the subjects proposed to be brought under consi- 

 deration in the present paper. In such cases I presume there would be 

 no perfect reversibility in the process ; and if so, this would of itself be a 

 criterion sufficing to separate them from the proper cases of boiling or 

 condensing at present intended to be considered. If, now, the fluid sub- 

 stance in the rarer of the two states (that is, in what is commonly called 

 the gaseous state) be still further rarefied, by increase of temperature or 

 diminution of pressure, or be changed considerably in other ways by 

 alterations of temperature and pressure jointly, without its receiving any 

 abrupt collapse in volume, it will still, in ordinary language and ordinary 

 mode of thought, be regarded as being in a gaseous state. Remarks of 

 quite a corresponding kind may be made in describing various conditions 

 of the fluid (as to temperature, pressure, and volume), which would in 

 ordinary language be regarded as belonging to the liquid state. 



Dr. Andrews (Phil. Trans. 1869, p. 575) has shown that the ordinary 

 gaseous and ordinary liquid states are only widely separated forms of the 

 same condition of matter, and may be made to pass into one another by a 

 course of continuous physical changes presenting nowhere any interruption 

 or breach of continuity. If we denote geometrically all possible points of 

 pressure and temperature jointly, by points spread continuously in a plane 

 surface, each point in the plane being referred to two axes of rectangular 

 coordinates^ so that one of its ordinates shall represent the temperature 

 and the other the pressure denoted by that point, and if we mark all the 

 successive boiling- or condensing-points of temperature and pressure as a 

 continuous line on this plane, this line, which may be called the boiling- 

 line, will be a separating boundary between the regions of the plane cor- 

 responding to the ordinary liquid state and those corresponding to the 

 ordinary gaseous state. But, by consideration of Dr. Andrews's experi- 

 mental results, we may see that this separating boundary comes to an end 

 at a point of pressure and temperature which, in conformity with his lan- 

 guage, may be called the critical point of pressure and temperature jointly; 

 and we may see that, from any ordinary liquid state to any ordinary 

 gaseous state, the transition may be effected gradually by an infinite variety 

 of courses passing round outside the extreme end of the boiling- line. 



Now it will be my chief object in the present paper to state and support 

 a view which has occurred to me, according to which it appears probable 

 that, although there be a practical breach of continuity in crossing the line 

 of boiling-points from liquid to gas or from gas to liquid, there may exist, 

 in the nature of things, a theoretical continuity across this breach having 



