202 ELECTROMOTIVE FORCE. chap.
the heat absorbed in a galvanic element, its electromotive force, and the change of this with temperature. He succeeded also in calculating, on thermodynamic principles, the electro- motive force of certain concentration cells.
Nemst treated the subject more from a kinetic point of view, and, by means of the theory of osmotic pressure, cal- culated the electric forces associated with the transport of the ions. Here, too, the kinetic view gives us a better picture of the process, but the thermodynamic method gives more trustworthy results. Xemst showed how, by ' the kinetic method, we can calculate the single electromotive force at the surface of contact of two liquids, whilst by the other method only the total eflfect can be obtained.
Galvanic Elements. — These may consist of a com- bination of conductors of the first and second class, metals and electrolytes, or, as in the liquid cells, only of electro- lytes. It is true that in the liquid cells there are always places of contact between metals and electrolytes, but these are so arranged that they exactly balance each other. Liquid elements are of great interest, because Nemst first gave the mechanical description of the production of an electromotive force for them. They are not, however, practi- cally used as sources of electricity.
Of the so-called hydro-elements, the best kno\vn is the Volta pile —
Zn I H2SO4 I Cu,
in which zinc passes into solution and hydrogen separates at the copper pole. This is a type of the irrevei'sible elements. The liydrogen is evolved and the original condition is not re- established when a current is passed through the element in the opposite direction ; in this case copper is dissolved and hydrogen is evolved at the zinc pole when the current is passed from copper to zinc through the solution.
In the theoretical respect, the so-called reversible elements behave much more simply ; in these the electrodes are non- polarisable, i.e, surrounded by an electrolyte, the positive ion
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