Capillary and Electrical Phenomena, 283 



ratus. A branch current then traversed the apparatus, which 

 now acted as a decomposing cell, until the E. F. P. produced was 

 equal to the difference of potentials between P and Q. The 

 E. F. P. bears the same relation to the electromotive force of a 

 Daniell as the resistance P Q bears to the resistance of the entire 

 circuit. This relation may be deduced from the deflection of a 

 tangent-compass contained in this circuit. The ratio of the 

 surfaces of mercury in M and in B was intentionally taken very 

 small, in order that the electromotive force of polarization in M 

 need alone be taken into account ; for it is clear that a quantity 

 of electricity which is sufficient to produce in M any given hy- 

 drogen polarization would give on the surface B, which is many 

 thousand times as large, no appreciable oxygen polarization. 

 Thus was obtained from the indications of the compass the E.F.P. 

 in M, and from the indications of the cathetometer the simul- 

 taneous capillary constant. In order to reduce the E. F. P. in M 

 to zero, all that was necessary was to interpose a simple metallic 

 circuit between a and /3. The magnitudes to be measured are 

 not small. Thus the depression in a tube having a radius 

 = 0*32 millim. is 14 millims. for an electromotive force of 

 polarization = ; for an electromotive force equal to one Da- 

 niell it is 18*90 millims. ; the alteration in level is therefore 

 equal to 4*90 millims. =0*35 of the original depression. The 

 constant of capillarity, therefore, is equal to 30*4 for an electro- 

 motive force equal to ; and is equal to 40*6 for an electromo- 

 tive force of polarization equal to one Daniell. 



In order to measure the alterations in the constant of capilla- 

 rity with still greater accuracy, instead of the tube Gr G' an ex- 

 tremely hollow open point yy (fig. 2) was used, which was pre- 

 pared by drawing out one end of a glass tube. Mercury was 

 poured into this tube to such a height that it penetrated into the 

 fine point and partially filled it (fig. 2). The point dipped in 

 dilute sulphuric acid ; the bubble of air which at first occupied 

 the end was removed by pressing out some mercury. There was 

 thus in the point a wetted hemispherical* meniscus M of about 

 y^q millim. radius, whose capillary pressure counterbalanced the 

 pressure of the mercury contained in the tube (750 millims. in 

 height). The dilute acid was also in contact with a second mass 

 of mercury, B, which, as before, was to serve as positive electrode ; 

 by means of platinum wires both masses could be connected with 

 the external poles a, /3. These poles were first placed in metallic 

 connexion with each other, whereby the E. F. P. in M was equal 

 to zero, and a microscope so placed in front of the point that 

 with a 220-fold magnification one of the cross-threads was in 



* The angle which mercury forms with glass under dilute sulphuric acid 

 rs always zero. 



