402 Dr. G. Gore on a Mew Method and 



" by plotting the quantities of dissolved substance as ordinates 

 to the degrees of electromotive force as abscissae, each sub- 

 stance or mixture of substances yielded a different curve of 

 electromotive force on uniformly changing the degree of 

 strength of its solution." And that "with a given voltaic 

 couple at a given temperature, the curve was constant and 

 characteristic of the substance/' The examples given in this 

 paper illustrate these statements, and the measurements were 

 made by the above method. 



Instead of employing a thermopile, the following plan with 

 the " voltaic balance" might be adopted : — 1st. Balance two 

 small zinc-platinum voltaic cells with distilled water in each, 

 and a galvanometer of 100 ohms resistance in the circuit, then 

 add gradually sufficient of the exciting substance to one of the 

 cells to just visibly upset the balance, and note the quantity 

 added. 2nd. Balance the cells with a very weak solution 

 containing one unit quantity of the substance in each cell ; 

 then add a sufficient excess of the substance to one cell to 

 again upset the balance, and note the amount of excess. 3rd. 

 Balance the cells with a two unit quantity solution, and upset 

 the balance again in like manner. And so on through such 

 a series of strengths of solution as may be desired to obtain 

 a curve. The proportions of excess of substance added to the 

 amount of water in the cell represent the relative amounts of 

 voltaic energy of the different strengths of liquid. 



This method is very much more sensitive than the one with 

 the thermopile, for whilst the latter only divides the range of 

 difference of potential between water and chlorine for example 

 ( = •753 volt), in practical working into about 15,000 parts, 

 the former divides it into 1200 millions. On the other hand, 

 whilst by the thermopile method the values of the unit of 

 measurement are equal throughout a series, those of the 

 numbers obtained by the voltaic-balance one vary with the 

 degree of strength of the solution. The following instance 

 illustrates this : — 



The liquid employed was a solution of potassium sulphate, 

 and the balancing couples were formed of zinc and platinum. 

 The table shows the degrees of strength of the solution, and 

 the proportions of additional salt required with each to upset 

 the balance. 



Strength of Solution. Excess of K 2 S0 4 . 



K 2 S0 4 . Water. K 2 S0 4 . Water. 



1*0 grain 155 grains. *30 grain = 1 in 51*6 



•5 „ „ „ -25 „ = 1 in 60-0 



•1 „ „ „ . '20 „ =1 m 77-5 



