i2 ELEMENTARY LESSONS ON [CHAP. in. 



osmose. The E.M.F. of the current varies with the 

 pressure and with the nature of the diaphragm. When 

 water was forced at a pressure of one atmosphere 

 through sulphur, the difference of potential was over 9 

 volts. With diaphragms of porcelain and bladder the 

 differences were only -35 and -01 volts respectively. 



225. Electro-Capillary Phenomena. If a hori- 

 zontal glass tube, turned up at the ends, be filled with 

 dilute acid, and a single drop of mercury be placed at 

 about the middle of the tube, the passage of a current 

 through the tube will cause the drop to move along 

 towards the negative pole. It is believed that the 

 liberation of very small quantities of gas by electrolysis at 

 the surface where the mercury and acid meet alters the 

 surface-tension very considerably, and thus a movement 

 results from the capillary forces. Lippmann, Dewar, 

 and others, have constructed upon this principle capillary 

 electrometers, in which the pressure of a column of liquid 

 is made to balance the electro-capillary force exerted at 

 the surface of contact of mercury and dilute acid, the 

 electro-capillary force being nearly proportional to the 

 electromotive-force when this does not exceed one volt. 

 Fig. 93 shows the capillary electrometer of Dewar. 

 A glass tube rests horizontally between two glass dishes 

 in which holes have been bored to receive the ends of 



Fig. 93- 



the tube. It is filled with mercury, and a single drop 

 of dilute acid is placed in the tube. Platinum wires to 

 serve as electrodes dip into the mercury in the dishes. 

 An E.M.F. of only ^ volt suffices to produce a measure- 



