WHAT IS ELECTRICITY ? 81 



metal rests directly upon that of tlie 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, v/hen 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 elec- 

 tro-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 sufllcient 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 mo- 

 lecular worlds may become better known to us ? 



When the world was evolved from the first nebulous stage, a por- 

 tion of the atoms remained more or less free in the gaseous state, 

 another portion became more or less limited in organic forms, and an- 

 other 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 super- 

 ficial 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 



VOL. XXVI. — 6 



