metal rests directly upon that of the other. The electrical current that arises is due to the difference of superficial energy manifested at the surfaces of the two junctions. We know that the action is on the surface, for the dimensions of the junctions do not affect the electro-motive force. Suppose that we should make the metals so thin that an ultimate molecule of iron should rest against an ultimate molecule of copper. Should we not arrive at a limit, at a definite temperature of the conversion of molecular vibration into electrical energy, and, also, when our theory is perfected, of the number of molecules along a linear line of copper against a linear line of zinc which can produce a current of electricity of a given strength? I have often thought that the jostling, so to speak, of these ultimate molecules of two metals at definite temperatures might form a scientific unit of electro-motive force in the future science of physical chemistry. Look at the great field for investigation there is in the measurement of what we call electro-motive force, both in voltaic electricity and in thermo-electricity. The astronomer measures the positions of the stars and their light, and tabulates the enormous volumes of results from year to year, in order to ascertain some great law or laws of the possible changes of the entire stellar universe—some sweeping onward through space. Is it not fully as important that, in our physical laboratories, we should organize our routine work, and provide some great generalizer, like Sir Isaac Newton, with sufficient data of electro-motive force, or, as I prefer to call it, the relations of superficial energy, in order that the relations between this energy and the ultimate motions of the molecular worlds may become better known to us?
When the world was evolved from the first nebulous stage, a portion of the atoms remained more or less free in the gaseous state, another portion became more or less limited in organic forms, and another portion were tightly compressed into solids more or less elastic. This elasticity is thought by some to be an evidence of very rapid motion through all these various aggregations of matter—or shall we say different manifestations of motion? for some also believe that our ideas of matter result merely from a perception of motion. Shall we affirm that there is some relation between elasticity and electricity? I do not think that we are prepared to do so, for some elastic bodies are good conductors and some are poor conductors of electricity. We can see dimly, however, that there is a great field in molecular physics, in which elasticity and superficial energy and difference of electrical potential shall be treated together.
I have tried various experiments upon the electro-motive force of alloys. By means of an alloy we can apparently modify the superficial energy at the surface of a solid. Thus, an alloy with a parent metal will give a varying electro-motive force. If we could be sure that an alloy is always a definite chemical composition, and not a more or less mechanical admixture, it seems as if we could get closer
