244 PHYSIOLOGICAL PHYSICS. [Chap. xxn. 



Indeed, the phenomena are exhibited when any 

 solid body is plunged into a liquid. At the surfaces in 

 contact the liquid is either raised or depressed, accord- 

 ing as the force of adhesion or cohesion sufficiently 

 predominates. 



The capillary electrometer. It was known 

 for some time that if a globule of mercury in dilute 



sulphuric acid was 



-"-v <' placed in contact with 



V c a / the positive pole of an 



element, while the 



Fig. 115. Capillary Electrometer. 



negative pole was in 



the sulphuric acid, on the passage of a current of 

 electricity the globule would move towards the nega- 

 tive pole. By interrupting and re-establishing the 

 current oscillations of the globule are produced, due to 

 changes in the surface tension of the mercury in contact 

 with the acid. The phenomena are more marked if 

 the mercury be contained in a capillary tube. A tube 

 suitable for the production of the phenomena was first 

 constructed by Lippmann. It " consists of a tube of 

 ordinary glass, one metre long and seven millimetres 

 in diameter, open at both ends, and kept in a vertical 

 position by a stout support. The lower end is drawn 

 into a capillary point, until the diameter of the 

 capillary is '005 of a millimetre. The tube is filled 

 with mercury, and the capillary point is immersed in 

 dilute sulphuric acid (1 to 6 of water in volume), and 

 in the bottom of the vessel containing the acid there 

 is a little more mercury." A platinum wire is con- 

 nected with the mercury in the tube, and another with 

 the mercury in the outer vessel containing the acid. 

 The capillary tube can be brought to the side of the 

 outer vessel, and viewed through a microscope, and an 

 oscillation perceived due to an extremely feeble cur- 

 rent. A modification of the instrument has recently 

 been devised by Professor McKendrick of Glasgow, 



