Electromotive Forces in the Voltaie Cell. aol 
III. Statements believed by the writer to be true, though not 
entirely orthodow. 
xii. A substance immersed in any medium tending to act 
upon it chemically is (unless it is actually attacked) at a 
different potential to the medium in contact with it ; positive 
if the active element in the medium is electro-positive, negative 
if the active element is electro-negative. 
xiii. The above difference of potential can be calculated 
approximately from the potential energy of combination 
between the substance and the medium; the energy being 
measured by compelling the combination to occur, and obser- 
ving the heat produced per amount of substance corresponding 
to one unit of electricity. 
xiv. In addition to this contact-force, due to potential 
chemical action or chemical strain, there is another which is 
independent of chemical properties but which seems to be 
greatest for badly-conducting solids, and which is in every 
case superposed upon the former contact-force, the two being 
observed together and called the Volta effect. Very little is 
known about this latter force except in the case of metals ; 
and in these it varies with temperature, and is small. In the 
case of non-metals it is often much larger than the chemical 
contact-force™. 
xv. The total contact-force at any junction can be expe- 
rimentally determined by measuring the reversible energy 
developed or absorbed there per unit quantity of electricity 
conveyed across the junction (practical difficulties, caused 
by irreversible disturbances, being supposed overcome). 
xvi. In a chain of any substances whatever, the resultant 
¥).M.F. between any two points is equal to the sum of the true 
contact-forces acting across every section of the chain between 
the given points (neglecting magnetic or impressed forces). 
xvii. In a closed chain the sum of the “ Volta forces,”’ 
measured electrostatically in any (the same) medium, is equal 
to the sum of the true contact-forces; whether each individual 
* T here assume, what I suppose is recognized as true, that what is 
known as frictional generation of electricity is really due to a contact-force 
between the substances rubbed—a force which is exceedingly great for 
insulators (see § 19). Davy seems to have held this view, from a note 
on p. 50 of his Bakerian lecture in 1806, cited before. 
+ These difficulties are, however, tremendous for most substances 
except metals. M. Bouty’s is the only attempt I know of to examine 
junction-energy between metals and solutions of their salts, which is the 
case next in simplicity to metals. Observe that the statement says energy, 
not heat only. 
2B2 
