296 Mr. G. Gore. On the Change of Potential of a [June 14, 



1st, the chemical composition of the liquid ; 2nd, the kind of positive 

 metal ; 3rd, to a less degree with the kind of negative metal ; 4th, 

 the temperature at the surface of the positive metal, and at that of 

 the negative one ; and 5th, with the kind of galvanometer employed. 



The order of the degree of sensitiveness or magnitude of the 

 " minimum-point " is manifestly related to that of degree of chemical 

 energy of the liquid, and, therefore, also to the atomic and molecular 

 weights of the dissolved substances, and to the ordinary chemical 

 groups of halogens. With certain exceptions, it is also distinctly 

 related to the amounts of chemical heat. The greater the degree of 

 free chemical energy of the dissolved substance, and the greater its 

 action upon the positive metal, the smaller the proportion of it 

 required to upset the balance. The proportion necessary for this 

 purpose probably represents a fixed amount of voltaic energy in all 

 cases, viz., the amount necessary to overcome the mechanical inertia 

 of the needle of the particular galvanometer employed. 



As the " minimum-point " of a chemically active substance dis- 

 solved in water is usually much altered by adding almost any soluble 

 substance to the mixture, measurements of that point in a number of 

 liquids at a given temperature with the same voltaic pair and galvano- 

 meter, will probably throw some light upon the state of combination 

 and degree of chemical freedom of substances dissolved in water. 



IL "On the Change of Potential of a Voltaic Couple by Varia- 

 tion of Strength of its Liquid." By G. Gore, F.R.S. 

 Received May 31, 1888. 



Having found a thermo-electric pile (see ' Birmingham Phil. 

 Soc. Proc.,' vol. 4, p. 130) convenient in detecting and measuring' 

 small changes of voltaic potential (' Roy. Soc. Proc.,' May 3rd, 1888), 

 I have taken advantage of that circumstance to measure by the 

 method of balance the above phenomenon in various liquids. 



The following are a few examples of measurements thus made of 

 the influence of varying quantities of different substances upon the 

 electromotive force of a voltaic couple composed of zinc and platinum 

 immersed in distilled water : — 



