524 report— 1884. 



xxix. A piece of zinc, half in air and half in water, causes no great 

 difference of potential between the air and the water (Thomson, Clifton, 

 Ayrton and Perry, &c), consequently air must behave much like water. 



xxx. If it makes the air slightly positive to the water, as it does 

 (Hankel), it may mean tbat the potential energy of combination of aiv 

 with zinc is slightly greater than that of water, or it may represent a 

 difference in the thermoelectric contact forces between zinc and air and 

 zinc and water, or it may depend on a contact force between air and water. 

 [If such a contact force between air and water exists, it is obviously of 

 great importance in the theory of atmospheric electricity, for tbe slow 

 sinking of mist globules through the air would render them electrical.] l 



xxxi. Condenser methods of investigating contact force no more avoid 

 the necessity for unknown contacts than do straightforward electrometer 

 or galvanometer methods ; the circuit is completed by air in the one case 

 and by metal in tbe other, and the E.M.F. of an air contact is more hope- 

 lessly unknown than tbat of a metal contact. 



xxxii. All electrostatic determinations of contact force are really deter- 

 minations of the sum of at least three such forces, none of which are- 

 knowable separately by this means. 



xxxiii. The only direct way of investigating contact force is by the 

 Peltier effect or its analogues. [Maxwell.] 



xxxiv. Zinc and copper in contact are oppositely charged, but are not at 

 very different potentials ; they were at different potentials before contact, 

 but the contact has nearly equalised them. 



xxxv. The potential of the medium surrounding them is, however, not 

 uniform. If a dielectric, it is in a state of strain ; if an electrolyte, it is 

 conveying a current. 



Size of Atoms. 



25. I may now claim to have accomplished my task, and terminate this 

 long paper ; but there are several interesting points which arise in con- 

 nection with Sir Wm, Thomson's deduction of a limit to the smallness of 

 atoms from contact data, and these I may be permitted to indicate. 

 Indeed, it evidently becomes a question whether or not his argument 

 remains quite valid if the chemical-strain view be taken of Yolta's force. 



Let us then inquire whether any modification has to be made in Sir 

 Wm. Thomson's argument, if the hypothesis set forth in this paper be 

 adopted. He says (virtually) take a number of plates of zinc and copper 

 of specified thickness, arrange them alternately like the leaves of a book 

 with the covers doubled right back, and then shut the book. Directly 

 they touched at one edge they became oppositely electrified and attracted 

 each other, and therefore did work as they approached. By making the 

 leaves numerous and thin enough, and shutting them up close enough, 

 any required amount of work can be thus done with given quantities of 

 metal, provided the thin plates retain the same properties as masses of 

 metal possess ; i.e., provided they are not only a few atoms thick. So far 

 there is no possible objection ; but Sir William proceeds to consider the 

 attraction as depending on the affinity of zinc for copper, and the work he 

 requires of his plates is that evolved in the formation of brass. But if we 

 regard the attraction as depending pn the difference of combustion 

 energies, Zn/O — Cu/O, we must, to keep the charge constant, not only 

 1 Cf. Lecture on 'Dust,' Nature, January 22, 1885. 



