32 6 



Dr. Ramsay. On the Critical State of Gases. [Apr. 22, 



When a liquid is heated under pressure it expands, and at the same 

 time evolves vapour. The vapour gains in specific gravity, while the 

 specific gravity of the liquid is rapidly diminishing. The critical point 

 is that point at which the liquid, oiving to expansion, and the gas, owing 

 to compression, acquire the same specific gravity, and consequently mix 

 with one another. From the first experiment it is seen that, on cooling, 

 the liquid contracts more rapidly than the gas, and consequently 

 separates as a mist through the whole of the tube, and, from its 

 gravity, separates at the lower half. The second experiment shows 

 that when the tube contains a small amount of liquid the space left 

 for gas is larger, and consequently more vapour must be given off by 

 the liquid before enough gas can be compressed till it acquires the 

 same specific gravity as the liquid; the temperature at which the 

 meniscus disappears is consequently higher. If the space left for gas 

 be smaller, the opposite is the case. From the third experiment it is 

 seen that it is possible to raise the critical point of a liquid, provided a 

 portion be continuously cooled. Probably the average temperature of 

 the two portions of the tube is about 220°. The fourth, fifth, sixth, 

 and seventh experiments demonstrate that by suitable means it is 

 possible to prevent, or rather to retard, the mixing of gas and liquid. 

 They then retain their several refractive indices. If, however, time 

 be allowed for diffusion through the capillary tube, the whole becomes 

 homogeneous, and the refractive indices of the fluids contained in 

 either portion of the tube are then identical. 



I shall not refer to Experiments until I have made a more thorough 

 investigation of the subject ; I venture to state, however, that it looks 

 probable that as- long as gas is being compressed, pressure rises 

 gradually with decrease of volume, whereas, even above their critical 

 points, liquids are comparatively incompressible. I am inclined to 

 think that carbonic anhydride, examined by Dr. Andrews, is abnormal 

 in this respect, but of this I am by no means certain. 



In conclusion, let me refer to a paper communicated to the Society 

 by Messrs. Hannay and Hogarth last October, entitled " On the Solu- 

 bility of Solids in Gases." Should the views of the subject suggested 

 by the above experiments be correct, it follows that these gentlemen 

 have observed nothing unusual, but merely the ordinary phenomenon 

 of solubility of a solid in a hot liquid. This view is borne out by 

 their own experiments. They found that on reducing pressure, that 

 is, on allowing the liquid to change to gas, the solid precipitated ; and 

 also on heating the tube containing the solution locally, that is, by 

 converting the liquid near the heated point into gas, precipitation 

 took place. I have taken the liberty of repeating one of their ex- 

 periments with a slight modification. 



A tube shaped like that used in Experiment 4, after a small frag- 

 ment of potassium iodide had been placed in the lower compartment, 



