304 



Mr. J. B. Hannay. 



contractile power, and such power cannot be observed except the 

 liquid have a free surface — that is, a surface bounded by another fluid 

 with which it is not miscible. It was found that all liquids such as 

 water, hydrocarbons, ethers, &c, however immiscible they may be at 

 ordinary temperatures, mix freely or act upon one another long before 

 the critical point of one of them is reached ; therefore liquids will not 

 serve to furnish a free surface. A gas, therefore, is the only substance 

 which will bear any pressure without becoming miscible ; and if it is 

 insoluble in the liquid (and all liquids have some gases insoluble in 

 them) we are provided with a substance to overlie the liquid which will 

 allow of a limiting surface being seeu at any pressure. This was the 

 method used. A quantity of pure dry hydrogen was placed over the 

 alcohol, and pressure applied, and the effect of rise of temperature 

 observed. It was seen that when the temperature rose to the critical 

 point the line dividing the alcohol from the mixture of alcohol vapour 

 and hydrogen became quickly indistinct, and was replaced by a broad 

 mark, indicating a gradual change in the refractive index of the fluid 

 when passing the place where the liquid surface had been, showing 

 that diffusion was taking place. On lowering the temperature before 

 much diffusion had taken place, the point where the liquid surface had 

 been became dim just as the temperature passed below the critical 

 temperature and the sharp limiting surface became re-established. 



The pressure was then increased by decreasing the volume of 

 hydrogen, and the experiment repeated many times at the new pres- 

 sure. The temperature must be raised much more slowly for these 

 observations than for the simple observation of the critical point with 

 alcohol alone, as in the latter case the upper and lower portions of the 

 fluid become of the same density, instantly obliterating the line of the 

 meniscus ; but when hydrogen is above the alcohol the line remains 

 although the alcohol be gaseous, until it is obliterated or broadened by 

 diffusion. Thus, on raising such a mixture to the critical temperature, 

 it is necessary to keep the temperature steady, to ascertain whether or 

 not diffusion will take place. If a rise of temperature is going on, 

 the thermometers will register a higher temperature than that at which 

 diffusion began. In .this way the following series of observations were 

 carried out : — 



Table IV. 



Alcohol with Hydrogen. 

 Manometers A" and C. 



T. 



T'. 



t. 





P. 



P. 



t". 



t'", 



230 



232 



80 



75 



220 



288 



17 



17 



229 



229 



75 



73 



221 5 



291 



20 



18 



231 



230 -5 



78 



70 



219 



287 



21 



19 



