when a difference of electrical potential is produced at the limiting surfaces. I have lately noticed a striking illustration of the modification of superficial energy by a difference of electrical potential. The experiment can be performed in this way: Fill the lower part of a glass jar with clean mercury, pour a saturated solution of common salt upon the mercury, hang in the salt solution a carbon plate, and connect this plate with a battery of four or five Bunsen cells; and, on connecting an iron wire with the other pole of the battery, touch the surface of the mercury. An amalgam will be speedily formed and chlorine gas given off. After a slight film of this amalgam has been formed on the mercury, remove the iron wire, and then immerse it slowly in salt-and-water. Even at a distance of six inches from the mercury, and far below the carbon electrode, the surface of mercury will be disturbed by the difference of electrical potential, and a commotion, which might be called an electrical storm, will be observed upon its surface. Now, these manifestations of what is called superficial energy—that is, the energy manifested at the surface of separation of any two media, and the effect of electricity upon this superficial energy—afford, it seems to me, much food for thought. There have always been two parties in electricity—one which maintains that electricity is due to the contact of dissimilar substances, and the other party which believes that the source of electrical action must be sought in chemical action. Thus, according to one party, the action of an ordinary voltaic cell is due to the contact, for instance, of zinc with copper, the acid or solution of the cell merely acting as the connecting link between the two. According to the other party, it is to the difference of chemical action on the two metals of the connecting liquid that we must attribute the rise and continuance of the electrical current. It has always seemed to me that these two parties are like the knights in the story, who stood facing opposite sides of a shield, each seeing but one side, one protesting that the shield was silver and the other that it was gold, whereas it was both silver and gold.
The electro-motive force of a voltaic cell is undoubtedly due to the intrinsic superficial manifestation of energy. When two dissimilar metals are placed in connection with each other, either directly or through the medium of a conducting liquid, the chemical action of the liquid brings new surfaces of the metals constantly in contact. Moreover, we have the difference of superficial energy between the liquid and the two metals. So that our expression for electro-motive force is far from being a simple one; it contains the sum of the several modifications of superficial energy at the surfaces of the two metals, and at the two boundaries of the liquid and the metals.
Let us turn now to the subject of thermo-electricity. Here we have again a development of electro-motive force by the mere contact of two metals, when the junctions of the metals are at different temperatures. There is no connecting liquid here, but the surface of one
