of Electrolytes on Volta-electromotive Force. 493 



in succession to a fixed amount of either nitric, sulphuric, 

 hydrochloric, hydrobromic, or hydriodic acid, the largest 

 amount of heat is evolved by the first quantity added ; 2nd, 

 by the diminishing amount of contraction which usually occurs 

 on adding successive equal portions of a salt to water ; and, 

 3rd, by the circumstance that on adding equal quantities in 

 succession, either of an acid or a salt, to water, the electric 

 conduction-resistance is much more largely diminished by the 

 first quantity added than by any of the succeeding ones. 



It is evident from these considerations that the property 

 in question, which I may term " the greater influence of first 

 quantities" is a very general one of electrolytes, and that the 

 four several phenomena mentioned are instances of it, and 

 are probably results of one common cause. The method of 

 examining this property by means of variations of electro- 

 motive force is very convenient, and the curves thus obtained 

 implicitly contain more detail of information than those ob- 

 tained by thermal measurements, because they represent more 

 perfectly integrated effects. 



The relatively greater increase of electric conductivity by 

 addition of the first unit quantity of acid or salt than by 

 any succeeding one, has been explained by the theory of 

 dissociation ; but it is also consistent with the simple 

 mechanical view that the molecules of the first added unit of 

 the dissolved substance have greater freedom and velocity of 

 motion ; the relatively greater electromotive force under those 

 conditions is also consistent with this idea. The increase of 

 electromotive force by addition of acid to water, and its de- 

 crease by addition of water to acid, admit of a very simple 

 explanation. The degree of electromotive force depends partly 

 upon the average velocity of the molecules of the liquid. 

 Strong acids possess greater molecular velocity and excite 

 greater electromotive force than water. By admixture of the 

 acid and water heat is evolved and the average molecular 

 velocity of each uniting substance is diminished. When we 

 add a little water to a large amount of acid, the average mole- 

 cular velocity of the mixture is less than that of the acid, 

 first, because water has less molecular velocity than the acid, 

 and, second, because heat is set free and molecular motion is 

 lost. But when we add a little acid to a large quantity of 

 water, the average molecular velocity of the mixture is greater 

 than that of water, because the molecular velocity of the acid 

 is so much greater than that of water that it more than com- 

 pensates for the loss of motion which occurs in the form of 

 heat during the act of mixing. 



