ELECTROMOTIVE FORCE OF CONTACT OF TWO LIQUIDS. 435 



Brown,* on the contrary, found very great differences by using 

 gases which attacked the metals, such as hydrochloric acid and 

 sulphuretted hydrogen. The arrangement used is that of Sir 

 W. Thomson's divided ring (1025). By placing the apparatus in 

 a bell jar, in which air and sulphuretted hydrogen are alternately 

 introduced, the needle deflects alternately to the left or right, until 

 the copper is covered with a blue layer of sulphide of copper. 

 But this permanent alteration of the surface of one of the metals 

 raises doubts as to the conclusions which can be drawn from these 

 experiments. 



After having discussed the results furnished by electrostatic 

 methods, Maxwell thus concludes : 



"These experiments seem to show that the agreement between the results 

 obtained by the ordinary methods for the electromotive forces of contact, and 

 those obtained by immersing the metals in water, or in any other oxygenated 

 liquid, is due less to the extreme smallness of the force between a metal and a 

 gas, or between a metal and an electrolyte, than to the fact that the properties 

 of air agree to a certain point with those of oxygenised electrolytes. And, in 

 fact, if the active component of the electrolyte is sulphur, the results change 

 altogether ; and the same is the case if the air is replaced by sulphuretted 

 hydrogen. " 



1030. M. Garbef has deduced from the properties of surface 

 tension, demonstrated by M. Lippmann, an ingenious method of 

 measuring the electromotive forces of contact. This method, as 

 Bichat and Blondlot J observe, may furnish absolute values inde- 

 pendent of the external medium. 



We have seen that the surface tension of mercury in contact with 

 a liquid, is a function solely of the difference of potential between 

 the liquid and mercury, and this difference is null when the surface 

 tension A is a maximum, for the capacity X of unit surface is then 

 zero. The electromotive force E, which must be introduced between 

 the two liquids is then equal, arid of opposite sign to the pre-existing 

 difference of potential. 



In a capillary electrometer in the ordinary condition, the external 

 electromotive force, which produces the maximum tension, is equal 

 to the difference of potential of contact Hg L of mercury with 

 acidulated water. If the acidulated water is replaced by a liquid I/, 



* J. BROWN. Phil. Mag. [5], Vol. vi., p. 142, 1878; [5], Vol. vn., p. 109. 

 1879. 



t Comptes rendus, Vol. xcix., p. 123. 1884. 

 J Comptes rendus, Vol. C., p. 791. 1885. 



F F 2 



