282 M. G. Lippmann on the Connexion between 



iron wire is fixed near it so as to dip in the acid and just 

 touch the edge of the drop of mercury. As soon as contact 

 has taken place, the drop performs regular vibrations which may- 

 last for hours. The resemblaDce between this phenomenon and 

 the movements of mercury electrodes (see Wiedemann, Galvanis- 

 mus, 1872, § 368) is striking, and the explanation is manifestly 

 the same. From the view hitherto received it would be as fol- 

 lows. The solution containing chromic acid would oxidize the 

 surface of the drop and thus flatten it. In contact with iron an 

 iron-mercury couple is formed. The current would reduce the 

 surface by electrolysis ; the drop would contract, and the contact 

 with the iron would be broken; the same succession of phenomena 

 would ensue as before, and so forth. By using a sufficiently 

 concentrated chromic acid solution these processes are actually 

 seen to take place ; but with a dilute solution the surface always 

 remains bright. Measurements have shown in fact that the po- 

 larization of mercury by hydrogen effects a contraction, and that 

 we need only remember the depolarizing action of chromic acid 

 to explain the above phenomenon. 



Experiments which I will now detail more completely have 

 shown that the capillary constant {superficial tension, coefficient of 

 Laplace's formula) at the cont act- sw face of mercury and dilute 

 sulphuric acid is an invariable function of the electromotive force 

 of the polarization at the same surface. 



I. Variation of the Capillary Constant with the Electromotive 

 Force of the Polarization. 



Measurements. — The apparatus consisted of a vertical ca- 

 librated glass tube, G G', which was connected at the bottom by 

 means of an india-rubber tube with a reservoir of mercury, A 

 (Plate II. fig. 1). The mercury rose thus in the tube G G', but 

 underwent there a capillary depression which was measured with 

 the cathetometer, and from which the capillary constant could 

 be determined in the ordinary manner. The upper part of the 

 glass tube was filled with dilute sulphuric acid (one eleventh of 

 the volume of acid), which wetted the meniscus of the mercury M, 

 and was continued by the glass siphon, H, into the glass vessel, 

 B_, which also contained dilute sulphuric acid. At the bottom 

 of the vessel was a layer of mercury, B, to serve as a second 

 electrode. The capillary depression of the mercury in the tube 

 G G' was, of course, corrected for the pressure of the dilute acid. 

 In order to evoke in M a known E. F. P. (Electromotive Force 

 of Polarization), the two masses of mercury (namely, that in B 

 and the mass A M) were respectively connected with two points, 

 P and Q, of the circuit of a DanielPs element by means of the 

 platinum wires a, ft, which may be called the poles of the appa- 



