524 



hki'ort — 1884. 



IWl 



xxix, A picco of zinc, half in air and half in water, causes no'pfreat 

 (liireronco of potential between the air and the water (Thomson, Clii'ton, 

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



XXX. If it makes the air slightly positive to the water, ns it doen 

 (Tlunkel), it may mean that the potential energy of combination of air 

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

 difference in the thermoelectric contact forces between zinc and air aTid 

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

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

 j^roat importance in the theoiy of atmospheric electricity, for the slow 

 linking of mist globules through the air would render thora electrical.] ' 



xxxi. Condenser methods of investigating contact force no more avoid 

 the necessity for unknown contacts than do straightforward electromeio: 

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

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

 lessly unknown than that of a metal contact. 



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

 rainations of the sum of at least three such foi-ces, none of which are 

 knowable separately by this means. 



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

 Peltier effect or its analogues. [Maxwell. J 



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

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

 but the contact has nearly equalised them. 



xxxy. The potential of the medium surrounding them is, however, nois 

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

 conveying a current. 



Size of Atoms. 



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

 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 pei'mitted to indicate. 

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

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



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

 Wm. Thomson's ai-gument, 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 euougli- 

 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 tiie 

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

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

 regard the attraction as depending on the difference of combnsboa 

 energies, Zn/0 — Cu/0, we must, to keep the charge constant, not only 

 * CI". IvGCture on ' Dust,' Naixirc, Januarj' 22, 1885. 





