480 



Mr. J. B. Hannay. 



[June 10, 



many organic compounds cannot be entirely freed from impurity, as 

 they retain it even on repeated distillation. In the following experi- 

 ments, therefore, such organic compounds were never used, and only 

 perfectly anhydrous alcohol, or carbon disulphide,* or gases which can 

 be obtained anhydrous, C0 2 , S0 2 , and NH 3 , being chosen. The ap- 

 paratus used for obtaining pressure was that described in a former 

 paper (" Proc. Roy. Soc.,"rTo. 201, 1880, " On the Solubility of Solids 

 in Gases"). In order to determine, then, whether increased pressure 

 applied above the critical point, would have the effect of reducing the 

 gas to a liquid, as might easily be supposed, since the rates of expansion 

 of gas and liquid become alike at the critical point, a new form of ex- 

 periment was resorted to. It had been noticed that it was easy to 

 determine whether the tube were filled with liquid or gas, by simply 

 reducing the pressure somewhat quickly, when, if there were liquid 

 present, it boiled, while if the contents were entirely gaseous, simple 

 expansion was the result. The boiling only takes place when the 

 pressure is reduced so far as to be a little under the vapour pressure at 

 that temperature, in other words, boiling cannot be observed, unless 

 there exists a free surface, and this free surface cannot be obtained with 

 the liquid alone above the " critical pressure." By the introduction of 

 a quantity of hydrogen gas over the liquid, a free surface is obtained 

 at any pressure, and the mixture of hydrogen and alcohol vapour being 

 of so much less density than the alcohol, it remains divided from it by 

 a line of demarcation for some time after the latter is undoubtedly 

 gaseous. Now, let us see what takes place on lowering the pressure. 

 When the temperature is even only 1°C. below the critical point, when 

 the pressure is sufficiently reduced, the alcohol boils, showing that it 

 still has cohesion, but if the temperature be 1° above the critical point, 

 the fluid only expands, and no boiling is seen at any pressure, from 50 

 up to 200 atmospheres. Here the fluid above the critical point has 

 just as free a surface as below it, and we see that the last trace of the 

 liquid condition has disappeared. The line dividing the mixture of 

 hydrogen and alcohol vapour from the pure alcohol is quite sharp, for 

 a short time, and on altering the pressure, it moves up and down 

 quite freely, and possesses exactly the same appearance and properties 

 as hydrogen over carbon dioxide in a bell-jar. f 



Thus we see that the liquid state seems to come to an end, and the 

 gaseous state to supervene quite independent of pressure. 



* Even this, in later experiments, has been found to dissociate slightly. (J une 14, 

 1880.) 



f The experiments must be done quickly, before the alcohol has time to dissolve 

 the hydrogen to any great extent, and the temperature must be equal all through the 

 apparatus, otherwise mixture takes place and these phenomena are not seen. Experi- 

 ments on mixtures obtained by shaking are in progress, but in these the critical 

 point is much altered. 



