TRANSACTIONS OP THE SECTIONS. 



31 



change either waj^ is perfectly reversible. When we thus haye two different states 

 present together in contact with one another, we have a perfectly obvious distinc- 

 tion, and we can properly continue to call one of them a liquid state and the other 

 a gaseous state of the same matter. The same two names may also reasonably be 

 applied to regions or parts of the fluid state extending away on both sides of the 

 shai-p ar definite boundary, wherever the merging of the one into the other is little 

 or not at all apparent. H we denote geometrically aU possible points of tempera- 

 ture and pressure 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 pressure and the other the temperature de- 

 noted by that point, and if we mark all the successive boiling- or condensino-- 

 points of pressure and temperature as a continuous line on this plane, this lin^e, 

 which may be called the boilimf-line, will be a separating boundary between the 

 regions of the plane coiTespoudiug to the ordinary liquid state and those corre- 

 sponding to the ordinary gaseous state. But by consideration of Dr. Andrews's 

 experimental results (Phil. Trans. 1869), we may see that this separating boun- 

 dary comes to an end at a point of temperature and pressure which, in conformity 

 with his language, 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 gradually efiected by an infinite variety of 

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



Fig. 1 is a_ diagram to illustrate these considerations and some allied consider- 

 ations to which they lead in reference to transitions between the three states, the 



Fig. 1. 



gaseous, the liquid, and the solid. This figure is intended only as a sketch to illustrate 

 principles, and is not drawn according to measurements for any particular substance, 

 though the mam features of the curves shown in it are meant to relate in a general 

 way to the substance of water, steam, and ice. A X and A Y are the axes of co- 

 ordinates for pressures and temperatures respectively ; A, the origin, being taken 

 as the zero for pressures and as the zero for temperatures on the Centigrade scale. 

 Ihe curve L represents the boiling-line. This terminates towards one direction in 

 the critical point E ; it passes in the other direction to T, the point of pressure 



