370 Professor W. Ramsay [May 8, 



given to the ether is greater than the critical volume. On increasing 

 the volume, you see that it boils away, and evaporates completely ; 

 and also what happens if the volume be somewhat less than the 

 critical volume ; it then expands as liquid, and completely fills the 

 tube. It is only at a critical volume and temperature that the ether 

 exists in the state of blue cloud, and has its critical pressure. If the 

 volume be too great, the pressure is below the critical pressure ; if 

 too small, the pressure is higher than the critical pressure. 



Still one more point before we dismiss this experiment. At a 

 temperature some degrees below the critical temperature, the menis- 

 cus, i. e. the surface of the liquid is curved. It lias a skin on its 

 surface ; its molecules, as Lord Kayleigh has recently exj)lained in 

 this room, attract one another, and it exhibits surface tension. Raise 

 the temperature, and the meniscus grows flatter ; raise it further, and 

 it is nearly flat, and almost invisible ; at the critical temperature it 

 disappears, having first become quite flat. Surface-tension therefore 

 disappears at the critical point. A liquid would no longer rise in a 

 narrow capillary tube ; it would stand at the same level outside and 

 inside. 



It was suggested by Professor James Thomson and by Professor 

 Clausius, about the same time, that if the ideal state of things were to 

 exist, the passage from the liquid to the gaseous state should be a 

 continuous one, not merely at and above the critical point, but below 

 that temperature. And it was suggested that the curves shown in the 

 figure, instead of breaking into the straight line of vapour-pressure, 

 should continue sinuously. Let us see what this conception would 

 involve. 



On decreasing the volume of a gas, it should not liquefy at the 

 point marked B on the diagram, but should still decrease in volume 

 on increase of pressure. This decrease should continue until the 

 j)oint E is reached. The anomalous state of matters should then 

 occur, that a decrease in volume should be accomj^anied by a decrease 

 of pressure. In order to lessen volume, the gas must be exposed to a 

 continually diminishing pressure. But such a condition of matter is 

 of its nature unstable, and has never been realised. After volume 

 has been decreased to a certain point F, decrease of volume is again 

 attended by increase of pressure, and the last part of the curve is 

 continuous with the realisable curve representing the compression of 

 the liquid above D. 



Dr. Sydney Young and I succeeded by a method which I shall 

 briefly describe in calculating the actual position of the unreali sable 

 portions of the curve. They have the form pictured in the figure 

 (shaded portion). The rise from the gaseous state is a gradual one ; 

 but the fall from the liquid state is abrupt. 



Consider the volume 14 cubic centimetres per gram on the figure. 

 The vertical equivolume line cuts the isothermal lines for the 



