8o THE POPULAR SCIENCE MONTHLY. 



when a difference of electrical potential is produced at the limiting 

 surfaces. I have lately noticed a striking illustration of the modifica- 

 tion of superficial energy by a difference of electrical potential. The ex- 

 periment 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 dis- 

 turbed 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 ordi- 

 nary 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 supei'ficial manifestation of energy. When two dissimilar met- 

 als 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 tem- 

 peratures. There is no connecting liquid here, but the surface of one 



