150 Mr. J. W. Clark on the Behaviour of Liquids 



slowly cooled, or even when somewhat rapidly cooled in an 

 external tube of large diameter, the liquid condenses in greater 

 quantity in the external tube than in the capillary tube, and 

 hence the liquid has to rise in the capillary tube before it 

 reaches the leyel of the liquid outside. This is easily seen 

 to be the case from the shape of the condensing cloud in an 

 external tube 20 millims. in diameter. It will be subsequently 

 seen that the resistance which the narrow tube offers to the 

 flow of the liquid through it, is at least yery closely connected 

 with the cause of the depression *. 



When the tubes are yery slowly heated in the apparatus 

 already described, the aboye phenomena are considerably 

 modified. The volume occupied by the same liquid before its 

 vaporization is then seen to be far greater than that occupied 

 by it when rapidly heated. To illustrate this the volume of 

 ether in a certain tube at the ordinary temperature was 

 roughly 25. Rapidly heated, the volume at which the liquid 

 disappeared was between 35 and 40; and at a volume of about 

 45 the last traces of black cloud had gone and the contents 

 of the tube appeared perfectly homogeneous. Slowly heated 

 when the liquid had expanded to a volume of 52, the meniscus 

 was slightly, but perceptibly, concave, and disturbed by a 

 rising bubble of gas. The enclosed capillary tube was not 

 long; and the liquid expanded until it reached the top, and 

 then poured into the tube and filled up the existing depression. 

 Indications that a higher temperature is required for this in- 

 creased expansion of the liquid have been observed; but 

 whether the temperature at which the liquid disappears is the 

 same for rapid and slow heating has not yet been satisfactorily 

 determined. Continuing to heat, the liquid expands, and the 

 surface is reduced to a thin and often waving line, obliterated by 

 further expansion, or lost amongst the frequent striae. Under 

 these circumstances the volumes of disappearance and reap- 

 pearance are nearly equal, and the corresponding temperatures 

 sometimes differ by less than o, l C. A very slight sudden 

 rise of temperature, when the liquid has expanded beyond the 

 volume at which it disappears under rapid heating, suffices to 

 replace the defined surface by the black ill- defined one before 

 described; but, apparently, when this expansion has proceeded 

 too far, the surface then becomes broad and ill-defined, but not 

 black. It then passes from the liquid through this ill-defined 

 state without expansion; and the liquid in a rather wide capil- 

 lary tube is then seen to be level with that outside it. Slowly 

 cooled, the tube frequently becomes uniformly filled with striae, 



* For this idea I am indebted to a suggestion made to me by my 

 riend Mr. Eagles. 



