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Mr. G. Gore. The Minimum-point of [June 14, 



June 14, 1888. 



The Right Hon. the EARL OF ROSSE, Vice-President, in the 



Chair. 



The Right Hon. John Hay Athol Macdonald (Lord Advocate), 

 Mr. Thomas Andrews, Mr. James Thomson Bottomley, Mr. Charles 

 Vernon Boys, Professor Arthur Herbert Church, Professor Charles 

 Lapworth, Professor William Ramsay, Mr. Thomas Pridgin Teale, 

 Mr. William Topley, Professor Henry Marshall Ward, and Mr. William 

 Henry White were admitted into the Society. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read : — 



I. " The Minimum-point of Change of Potential of a Voltaic 

 Couple." By G. Gore, F.R.S. Received May 26, 1888. 



In a previous communication on " The Effect of Chlorine on the 

 Electromotive Force of a Voltaic Couple " (' Roy. Soc. Proc.,' 

 May 3rd, 1888), I described a phenomenon which I now venture to 

 term the " Minimum-point of Change of Potential of a Voltaic 

 Couple." In that description a " thermo-electric pile " is mentioned 

 as having been used for the purpose of balancing the electromotive 

 force of the couple, whilst finding the " minimum-point of change." 

 As very few persons possess a thermo-electric pile suitable for the 

 purpose, I have devised and employed the following arrangement by 

 means of which the use of the pile may be dispensed with. 



Take a voltaic couple, composed of an unamalgamated strip or 

 stout wire of zinc or magnesium (the latter is usually the best), and 

 a small sheet of platinum, immersed in distilled water ; balance its 

 electric potential through an ordinary galvanometer by that of a pre- 

 cisely similar couple composed of portions of the same specimens of 

 the same metals, immersed the same moment as the other pair in a 

 separate quantity of the same water, and gradually add to one of the 

 two cells sufficiently small and known quantities of an adequately 

 weak solution of known strength in a portion of the same water, of 

 the substance to be used, until the balance is upset, and take note of 

 the proportions of the substance and of water then contained in that 

 cell. It is more easy to successively dilute than to successively 



