408 Scientific Proceedings, Royal Duhli7i Society. 



gas ; indeed, at one stage, the rise of pressure witli dimiuution of volume is 

 barely perceptible. At a slightly higher temperature the opalescence is never 

 visible ; and the compressibility is never so great as at the critical point. 



When a liquid is converted into vapour, work is done and heat is required 

 {a) to overcome the mutual attraction of the molecules, and (b) to increase the 

 volume against tlie external pressure. The heat of vaporization— i.e., tlie 

 amount of heat required to convert 1 gram of liquid into saturated vapour — 

 diminislies as the temperature rises, and must necessarily vanish at the 

 critical point, since at this temperature the volumes of saturated vapour and 

 liquid are equal, or, more correctly, liquid and vapour are identical. The 

 critical temperature is that temperature at wliieh the heat of vaporization 

 vanishes. 



When the temperature of a sealed tube containing a liquid and its satu- 

 rated vapour is gradually raised, one of three things will liappen — 



1. If tlie amouTit of liquid is smaller than a definite minimum, the loss 

 of volume due to evaporation — above a certain temperature at any rate — will 

 be greater than the gain due to expansion; and the liquid phase will disappear 

 before the critical temperature is reached. 



2. If the amount of liquid is greater than a definite maximum, tlie gain 

 in volume due to expansion will be greater than the loss due to evaporation ; 

 and the liquid will completely fill the tube before the critical point is reached. 



3. If the amount of liquid is between these limits, the two phases will 

 persist up to the critical point, although the meniscus may approach the 

 bottom or top of the tube. At low temperatures the meniscus is concave ; but, 

 as the critical temperature is approached, it becomes flatter and less distinct ; 

 and at the critical temperature it disappears, and its place is taken by a band 

 of mist which gradually spreads upwards and downwards through the tube. 

 At a slightly higher temperature the mist disappears. Two determiuations 

 of the critical temperature of a given substance by this method should not 

 differ by 0-1°. 



"With the pressure-apparatus the volume can be altered, and a still more 

 accurate determination of the critical temperature may be made. By 

 suddenly increasing or diminishing the volume to a very small extent, a 

 temporary minute fall or rise of temperature may be brought about. The 

 critical temperature is taken to be that at which no meniscus is visible 

 when the volume is constant, but at which the meniscus just appears for 

 a moment when the volume is very slightly increased. The difference 

 between two determinations by this method sliould not exceed 0'05°. 



Some observers have stated (1) that the temperature at which the meniscus 

 disappears when a tube is gradually heated differs from that at which it 



