154 On Liquids and Gases near their Critical Temperatures. 



shorter period merely sufficed to diminish the depression. 

 Probably closely connected with this action is the gradual 

 decrease Quincke * has observed that time produces in the 

 form of a bubble of gas in a liquid, and of a drop of mercury. 

 To this class of facts are also nearly related the decrease in 

 the intensity of Quincke's f electrical diaphragm-currents, 

 and that -which J I have shown to take place in the electro- 

 motive force produced when water is forced through capillary 

 tubes. Elster § has recently extended the observation to a 

 similar variation in the electromotive force set up by liquids 

 flowing over the surfaces of solids. Dornfl has investigated 

 at some length the cause of this action in the case of tubes, 

 and has shown that it is capable of modification in various ways, 

 some of which appear capable of exercising a corresponding 

 control over the above-described depression of a liquid in a 

 capillary tube at a temperature near the critical. 



It is proposed to continue the still incomplete portions of 

 this inquiry in a paper to the Society next session. 



In conclusion I beg leave to express my thanks to Professor 

 McLeod, not only for having advised me to extend my obser- 

 vations to higher temperatures than those at first employed, 

 but also for the willingness he has always shown to aid me 

 with valuable suggestions. 



Summary of Contents. 



1. When a tube enclosing a capillary tube dipping into 

 alcohol, ether, or sulphurous anhydride is heated, the liquid 

 sinks in the capillary, and rises by expansion in the outer 

 tube. Between 2° and 3° C. below the critical temperatures of 

 these liquids both surfaces become level; and on continuing 

 to heat, the concave meniscus in the capillary tube is seen 

 below that in the external tube. The extent of this de- 

 pression depends on the diameter &c. of the capillary tube, 

 and on the nature of its internal surface. When the end of 

 a capillary tube dips very slightly below the surface of the 

 liquid, it is level in the capillary and external tubes at the 

 disappearance of the liquid. 



2. In some capillary tubes the liquid is not depressed, but 

 disappears at the level of the liquid in which they are immersed 

 on first heating. Once heated, long contact between liquid 

 and tube is necessary to prevent the formation of the depression 



* Pogg. Ann. Bd. clx. S. 576. f Ibid. Bd. ex. S. 56. 



X Ibid. 1877, S. 345. 



§ Inauyural-Dissertation iiber die in freien Wasserstrahlen auftretenden 

 electromotorischen Krafte. Leipzig, 1879. 



|| Wiedemann's Ann. Bd. ix. 1880, S. 523. Compare also Helmholtz, 

 Wied. Ann, vii. p. 337 (1879). 



