and Gases near their Critical Temperatures. 151 



and the light transmitted through the experimental tube by the 

 paraffin lamp before described is observed to become gradually 

 redder and redder. This is succeeded by the formation of a 

 whitish incipient cloud, which finally precipitates in visible 

 particles, often throughout the whole tube. The liquid con- 

 tracts from the first moment of its condensation until it regains 

 its original volume. Slow and regular cooling seems more 

 difficult to attain than the corresponding conditions on heating. 



When the external tube contains rather less ether than 

 the above — that is, when about one third filled, and rapidly 

 heated, the liquid expands and passes into gas in the usual 

 way. Very slowly heated the liquid also expands; but after 

 reaching a certain maximum volume, it very gradually dimi- 

 nishes and evaporates away. If the volume of liquid at its 

 point of maximum expansion happens to be such that the 

 liquid in the external and capillary tubes are almost level, the 

 meniscus in the capillary tube shows a slight tendency to in- 

 crease its height above the surface when the contraction 

 commences. A slight sudden rise of temperature produces a 

 result similar to that described in the last case. On cooling, 

 the liquid in such a tube undergoes a momentary and almost 

 inappreciable increase of volume just after condensation, and 

 then contracts to its initial volume as in the previously de- 

 scribed tubes. If a tube contains still less ether than the last 

 mentioned, the liquid undergoes a more marked increase of 

 volume when it first condenses; and this continues longer 

 before the normal contraction sets in. 



Very near the critical temperature the density of liquid 

 and gas are almost equal; and hence it is that the meniscus in 

 the capillary tube may remain depressed for as much as an 

 hour and a half, although very slowly following the upward 

 motion of that in the external tube. The meniscus in the 

 capillary tube usually fades away a little before the surface 

 ceases to be visible in the external tube. This may be due 

 to its being under a slightly lower pressure. 



The meniscus in a wide capillary tube is observed below the 

 surface of the liquid in the external tube before that in a 

 capillary of small diameter ; and rapidly heated, the depression 

 usually remains greater in the wide tube until the surface 

 ceases to be visible. This disappearance frequently takes 

 place first in the small, then in the larger capillary, and lastly 

 in the external tube ; when the heating is sufficiently slow, 

 the depression becomes greatest in the tube of small diameter. 

 Slow or rapid heating and slow cooling alike show the de- 

 pression is greater in a tube roughened by hydrofluoric acid 

 than it is in a smooth one of the same size. "When the 



