1880.] State of Fluids at their Critical Temperatures. 483 



tube is made to rise to about D, where it rests against the upper side 

 of the slanting part of the tube. When the temperature and 



pressure required were reached the tube was gently tapped, when 

 the bubble of nitrogen passed round the bend up into the fluid at A. 

 If the fluid be in the liquid state, the bubble shows a meniscus ; but if 

 the fluid be gaseous, the bubble instantly diffuses into the inclosed space, 

 showing that the fluid has no cohesion or has assumed the gaseous 

 state. The same results were obtained as before. When the tempe- 

 rature was below the critical point the contents of the tube were liquid, 

 and when over that temperature the reaction was always gaseous, 

 notwithstanding the variations of pressure. 



I think that we have in these experiments evidence that the liquid 

 state ceases at the critical temperature, and that pressure will not 

 materially alter the temperature at which the cohesion limit occurs. 

 Dr. Andrews has, indeed, indicated his belief that such was the case ; 

 but as far as I am aware, there has been till now no direct experi- 

 mental proof of the fact. When the line dividing the fluid from the 

 gas, and the difference in height outside and inside the capillary tube 

 disappear, it cannot be said that this proves the total absence of 

 cohesion, but only that the cohesion of the two portions of the fluid 

 becomes equal; and it is only if the gas or vapour has no cohesion that 

 we can say that the portion which formed the liquid has none either. 

 But by the introduction of a third substance, which, when the experi- 

 ments are done quickly, acts only on one side, it seems, from the 

 results observed, that the condition of the surface of demarcation 

 depends mainly, if not entirely, upon the lower or liquid portion of 

 the fluid. The critical points of liquids are thus truly the absolute 

 boiling points, as has been aptly said, and are closely comparable to 



vol. xxx. 2 M 



