496 Mr. 1a. Sabine on the Motions produced 



dueed in 1873 by Mr. G. Lippmann under the name of a 

 c< capillary electrometer," in which similar circulations set up 

 in themercury produce, of course, a similar alteration of level, 

 but in explanation ofwhich Mr. Lippmann reproduces Draper's 

 theory of capillarity. In another form of Mr. Lippmann's 

 "electrometer," a column of mercury is supported vertically 

 in a tube, the bottom of which is drawn out to a capillary bore 

 and dips into dilute sulphuric acid. When the capillary 

 niveau is deoxidized, the circulation which is set up in the 

 mercury tends to cause it to recede. This recession is op- 

 posed and finally balanced by the superincumbent column. 

 When the capillary end of the tube is slightly conical, which 

 is generally the case, it is observed that on breaking the cir- 

 cuit the niveau of the mercury returns to very nearly its original 

 position. These motions require a very small current to produce 

 them, the different states of oxidation of the two mercury 

 surfaces, when the circuit is simply closed without a battery, 

 being generally sufficient to cause a perceptible movement. 



The movement of the mercury in Draper's experiment was 

 limited by the weight of the column raised by it. When 

 a straight horizontal tube (capillary or otherwise) provided 

 with enlarged ends is used, the acid water in it being con- 

 nected with the battery, the mercury globule pumps very 

 little water past it, but marches itself onwards to the end of 

 the tube. With a tube whose bore was about O05 centim. dia- 

 meter, Sir C. TYheatstone found that a single Daniell cell caused 

 sufficient motion to take place, when 300,000 ohms resistance 

 were inserted in the circuit, to give readable signals*. 



If an inequality could, in any way, be produced between 

 the capillary constant at two points of a mass of mercury, 

 without being due to an alteration of the chemical constitution 

 of the surface, such an inequality might set the mercury in 

 movement. But by no means that I am aware of can the 

 capillary constant of mercury, as long as the chemical nature 

 of its surface continues unchanged, be made inconstant. To 

 a common cause, displacement by chemical action, are due 

 both change of surface-tension and locomotion ; and the one 

 effect cannot, in any sense of the word, be said to be the cause 

 of the other. 



* This experiment is, in effect, the same as that devised by Erman (in 

 1809), which first suggested to Sir C. Wheatstone the idea of constructing 

 a mercury receiving instrument for telegraphic purposes. Erman offers 

 the following explanation of his experiment : — " The elongation and pro- 

 gressive motion of the cylinder of mercury in the tube may be strictly ex- 

 plained by the mere augmentation of adhesion and by the introduction of 

 the water into the capillary space which results from it.'' Erman also sus- 

 pected u an attraction acting at a distance." 



