358 Prof. Oliver Lodge on the Seat of the 
xxvil. When a current flows from zinc to acid, the energy 
of the combination which occurs is by no means accounted 
for by the heat there generated, and the balance is gained by 
the current ; hence at a zine-acid junction there must be a 
considerable E.M.F. (say at a maximum 2°3 volts). 
xxvili. A piece of zinc immersed in acid is therefore at a 
lower potential than the acid, though how much lower it is 
impossible precisely to say, because no actual chemical action ~ 
occurs. [If chemical action does occur, it is due to impurities, 
or at any rate to local currents, and is of the nature of a dis- 
turbance. | 
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 that the potential-energy of com- 
bination of air with zinc is slightly greater than that of water, 
or it may represent a difference in the thermoelectric contact- 
forces between zine and air and zinc and water, or it may 
depend on a contact-force between air and water. [If sucha 
contact-foree between air and water exists, it is obviously of 
great importance in the theory of atmospheric electricity, for 
the slow sinking of mist-globules through the air would 
render them electrical*. | 
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 
the other, and the H.M.F. of an air-contact is more hopelessly 
unknown than that of a metal-contact. 
xxxii. All electrostatic determinations of contact-force are 
really determinations 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. Zine 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 equalized 
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 1s conveying a current. 
* Cf. lecture on “ Dust,” Nature, 22nd January, 1885. 
