14:8 Mr. J. W. Clark on the Behaviour of Liquids 



three or four degrees of the critical ; the glass stop-cock (c) is 

 then closed, and the pressure of the expanding air in the flask 

 begins to raise the mercury until it meets the platinum tube 

 (e) of the regulator, which is previously screwed down till 

 within about 3 millims. of the surface of the mercury. Some 

 two or three hours later the critical temperature is reached. 

 The light from a paraffin lamp passes through a cylindrical lens, 

 and enters through one of the glass windows in the case en- 

 closing the apparatus, thus rendering the experimental tube 

 visible through the other window. The observations are made 

 with a cathetometer-telescope. 



Early in this inquiry it was found that so many circum- 

 stances influenced the depth to which the liquid was depressed 

 in the capillary tube, that two different tubes could be satis- 

 factorily compared only when enclosed in the same external 

 tube ; and if the effects of slow and rapid heating are to be 

 compared, the tubes should be of the same thickness of glass. 

 Small springs of thin platinum (S, fig. 2) fix the capillary tubes 

 vertically in the axis of the external tubes, which latter have 

 been employed of various diameters between 2 and 20 millims. 

 TVhen the slow heating-apparatus is used, the experimental 

 tube (enclosing the capillary tube) is fixed in the axis of a 

 tube of considerably greater diameter, which is then exhausted 

 of air and hermetically sealed (see fig. 3). Ether distilled 

 from calcic chloride has been the liquid most frequently em- 

 ployed ; but, so far as I am aware, all the results about to be 

 described are also obtained with alcohol, sulphurous anhydride, 

 and carbonic disulphide. Water attacks glass so rapidly that 

 it is difficult to ascertain what is taking place in the tube ; but 

 it probably forms no exception to the liquids mentioned. 



After the introduction of the capillary, the external tube is 

 filled with liquid, and the air removed as completely as 

 possible by repeated boiling, and the volume of the liquid is 

 reduced to a convenient extent. When such a tube contains 

 ether and is heated, the liquid sinks in the capillary tube and 

 rises in the outer, the expansion becoming more and more 

 rapid as the critical temperature is approached. About 2° C. 

 below this temperature the meniscus in the capillary tube 

 stands at the same level with that in the external tube. Both 

 surfaces are then distinctly concave. In the case of alcohol 

 the temperature at which this is observed is probably a little 

 more, and for sulphurous anhydride a little less, than 2° C. 

 below the critical temperature. The exact temperature is 

 affected by a number of circumstances : thus it is lower with 

 a wide capillary tube than a narrow one, and for the same 

 capillary tube it is also lower when this tube is deeply immersed 



