284 M. G. Lippmann on the Connexion between 



the tangent plane of the meniscus (fig. 3). To make a mea- 

 surement, a DanielFs element, for instance, was interposed be- 

 tween a and /3 : the column of mercury disappeared from the field 

 of view; and in order to adjust the mercury to the thread a new 

 pressure, which might be called the compensating pressure, had to 

 be exerted upon the column of mercury which supported the me- 

 niscus. This compensating pressure bears to the former pres- 

 sure the same ratio that the increase of the constant of capilla- 

 rity at the meniscus does to the former value of this magnitude. 

 This is clear from Laplace's formula, in which the curvature 

 must be assumed to be constant when the adjustment of the 

 meniscus is constant, and thus the pressure is proportional to 

 the constant of capillarity. In the example given the compen- 

 sating pressure amounts to 260 millims. of mercury (more than 

 one third of an atmosphere) when a DanielPs element is inter- 

 posed — that is, 035 of the former pressure (750). The constant 

 of capillarity has increased by 0*35 of its former value. 



The compensating pressure was exerted by means of com- 

 pressed air, produced and measured by means of an air-pump 

 and of a mercury manometer. Electromotive forces of polari- 

 zation which were equal to known fractions of a Daniell were 

 produced by the above-described method of branch currents. 

 It was thus found that to every value of the E. F. P. a perfectly 

 definite value of the constant of capillarity corresponds ; so that 

 from the one magnitude may be inferred the value of the other. 



In all of these experiments in which the circuit was closed (that 

 is, where between u and ft either a mere wire or a constant elec- 

 tromotive force was interposed), it was surprising to observe how 

 constant were the results — that is, the constant of capillarity 

 and the invariability of the position of equilibrium of the me- 

 niscus. We have been accustomed to certain " disturbances " 

 which occur in capillary experiments arranged in the ordinary 

 manner (that is, without completion of the electrical circuit), and 

 which, of course, occurred here also when a and j3 were insulated 

 from each other. These disturbances are as follows : (1) the 

 position of equilibrium is different according to the direction of 

 the previous motion of the mercury; (2) it may suddenly change 

 on agitation, as, for instance, by tapping ; (3) the position of 

 equilibrium may slowly change with time and only cease to 

 become displaced hours after. But when the circuit was closed 

 by interposing, for instance, a wire between a and /3, all irregu- 

 larities at once disappeared and they could not be reproduced ; 

 that is, the position of equilibrium became so constant that the 

 meniscus was at once adjusted to the cross- wire with a precision 

 which, in spite of the 220-fold magnification, left nothing to 

 be desired. Moreover the cause of these disturbances was dis- 

 covered (p. 290). 



