Electromotive Forces in a Voltaic Cell. 379 



The remarkable thing about the charges of atoms is, how- 

 ever, that they are all equal, and even for different atoms are still 

 the same, or differ only according to simple multiples of some 

 one absolute quantity; so that one atom may have three times 

 as much negative electricity as hydrogen has positive, but not 

 any fractional number. Such an atom may therefore combine 

 with three of hydrogen atoms, and thus be called a triad. 

 Another may be a dyad, &c. A given constant charge belongs 

 to every unit of affinity which the atom possesses ; and thus is 

 the fact of chemical equivalence stated, though not accounted 

 for. Faraday's law compels us to believe that atomic charges 

 are thus multiples of one definite electric quantity, but why 

 they should be so we are wholly unable to say. It looks as if 

 electricity were atomic as well as matter ; a sufficiently 

 startling idea, as Helmholtz says. 



Chemical affinity is thus due to the electrical attraction of 

 oppositely charged atoms ; not so much due to the attraction 

 of the atoms themselves. This latter kind of attraction Helm- 

 holtz would not indeed deny, and he considers it may account 

 for " molecular " modes of combination ; but he regards it as 

 much weaker than the electric forces, and as not effecting 

 definite chemical combination. 



We have ideas now concerning the size of atoms, and can 

 calculate roughly what an atomic charge is. The charge of 

 each atom is but small, but the aggregate charge of an appre- 

 ciable number of them is enormous. It can be easily reckoned 

 that if you take the opposite electricities out of a milligramme 

 of water and give them to two spheres a mile apart, those 

 two spheres will attract each other with a force of ten tons ! 

 Or, again, the electrical attraction of two atoms at any dis- 

 tance exceeds their gravitative attraction 71 thousand billion 

 times. Upon such charges as these even very feeble external 

 electrical influence may exert considerable force, and quite 

 overpower any other kind of chemical affinity. 



Plunge, therefore, two platinum plates into the liquid and 

 keep them at some slightly different potential, say a volt or 

 two : their surfaces are oppositely charged, and even this 

 feeble charge may be sufficient to tear asunder the atoms of 

 molecules which come close up to it"*. If it is not sufficient, it 



* It is easy to calculate the surface-density, and the difference of po- 

 tential, on the above hypothesis, needed to tear asunder oppositely charged 

 atoms attracting each other across molecular distance x. Let it be o-, and 

 let the charge of an atom be q; then 



Moreover we know that 19,320 electromagnetic units can decompose 18 

 grammes of water, which allows 536 X 3 X ] 10 electrostatic units of each 



