April T, 1 88 1 J 



NATURE 



539 



chemical force existed besides tliat of the electrical charges 

 which could bind all the pairs of opposite ions together, 

 and required any amount of work to be vanquished, an 

 inferior limit to the electromotive forces ought to exist, 

 which forces are able to attract the atoms to the elec- 

 trodes and to charge these as condensers. No pheno- 

 menon indicating such a limit has as yet been discovered, 

 and we must conclude therefore that no other force resists 

 the motions of the ions through the interior of the liquid 

 than the mutual attractions of their electric charges. 



On the contrary, as soon as an ion is to be separated 

 from its electrical charge we find that the electrical forces 

 of the battery meet with a powerful resistance, the over- 

 powering ot which requires a good deal of work to be 

 done. Usually the ions, losing their electric charges, 

 are separated at the same time from the liquid ; some of 

 them are evolved as gases, others are deposited as 

 rigid strata on the surface of the electrodes, hke 'gal- 

 vanoplastic copper. But the union of two constituents 

 having powerful affinity to form a chemical compound, as 

 you know very well, produces always a great amount of 

 heat, and heat is equivalent to work. On the contrary, 

 decomposition of the compound substances requires work, 

 because it restores the energy of the chemical forces, 

 which has been spent by the act of combination. 



Metals uniting with oxygen or halogens produce heat 

 in the same way, some of them, like potassium, sodium, 

 zinc, even more heat than an equivalent quantity of 

 hydrogen ; less oxidisable metals, like ccpper, silver, 

 platinum, less. We find therefore that heat is generated 

 when zinc drives copper out of its combination with the 

 compound halogen of sulphuric acid, as is the case in a 

 Daniell's cell. 



If a galvanic current passes through any conductor, a 

 metallic wire, or an electrolytic fluid, it evolves heat. Mr. 

 Frescott Joule was the first who proved experimentally 

 that if no other work is done by the current the total 

 amount of heat evolved in a galvanic circuit during a 

 certain time is e.xactly equal to that which ought to have 

 been generated by the chemical actions which have been 

 performed during that time. But this heat is not evolved 

 at the surface of the electrodes, where these chemical 

 actions take place, but is evolved in all the parts of the 

 circuit, proportionally to the galvanic resistance of every 

 part. From this it is evident that the heat evolved is an 

 immediate effect, not of the chemical action, but of the 

 galvanic current, and that the chemical work of the 

 battery has been spent in producing only the electric 

 action. 



If we apply Faraday's law, a definite amount of elec- 

 tricity passing through the circuit corresponds to a definite 

 amount of chemical decomposition going on in every 

 electrolytic cell of the same circuit. According to the 

 theory of electricity the work done by such a definite 

 quantity of eleetricity which passes, producing a current, 

 is proportionate to the electromotive force acting between 

 both ends of the conductor. You see therefore that the 

 electromotive force of a galvanic circuit must be, and is 

 indeed, proporuonal to the heat generated by the sum of 

 all the chemical actions going on in all the electrolytic 

 cells during the passage of the same quantitity of 

 electricity. In cells of the galvanic battery chemical forces 

 are brought into action able to produce work ; in cells 

 in ^yhich decomposition is occurring work must be done 

 against opposing chen.ical forces ; the rest of the work 

 done appears as heat evolved by the current, as far as it 

 is not used up to produce motions of magnets or other 

 equivalents of work. 



Hitherto we have supposed that the ion with its electric 

 charge is separated from the fluid. But the ponderable 

 atoms can give off their electricity to the electrode, and 

 remain in the liquid, being now electrically neutral. This 

 makes almost no difference in the value of the electro- 

 motive force. For instance, if chlorine is separated at 



the anode, it will remain at first absorbed by the liquid ; 

 if the solution becomes saturated, or if we make a vacuum 

 over the liquid, the gas will rise in bubbles. The electro- 

 motive force remains unaltered. The same may be 

 observed with all the other gases. You see in this case 

 that the change of electrically negative chlorine into 

 neutral chlorine is the process which requires so great an 

 amount of work, even if the ponderable matter of the 

 atoms remains where it was. 



The more the surface of the positive electrode is 

 covered with negative atoms of the anion, and the 

 negative with the positive ones of the cation, the more 

 the attracting force of the electrodes exerted upon the 

 ions of the liquid is diminished by this second stratum of 

 opposite electricity covering them. On the contrary, the 

 force with which the positive electricity of an atom of 

 hydrogen is attracted towards the negatively charged 

 metal increases in proportion as more negative electricity 

 collects before it on the metal, and the more negative 

 electricity collects behind it in the fluid. 



Such is the mechanism by which electric force is con- 

 centrated and increased in its intensity to such a degree 

 that it becomes able to overpower the mightiest chemical 

 affinities we know of If this can be done by a polarised 

 surface, acting like a condenser, charged by a very mode- 

 rate electromotive force, can the attractions between the 

 enormcus electric charges of anions and cations play an 

 unimportant and indifferent part in chemical affinity? 



You see, therefore, if we use the language of the 

 dualistic theory and treat positive and negative electrici- 

 ties as two substances, the phenomena are the same as 

 if equivalents of positive and negative electricity were 

 attracted by different atoms, and perhaps also by the 

 different values of affinity belonging to the same atom 

 with different force. Potassium, sodium, zinc, must have 

 strong attraction to a positive charge ; oxygen, chlorine, 

 bromine to a negative charge. 



Faraday very often recurs to this to express his convic- 

 tion that the forces termed chemical affinity and electri- 

 city are one and the same. I have endeavoured to give 

 you a survey of the facts in their mutual connection, 

 avoiding, as far as possible, introducing other hypotheses, 

 except the atomic theory of modern chemistry. I think 

 the facts leave no doubt ihat the very mightiest among 

 the chemical forces are of electric origin. The atoms 

 cling to their electric charges and the opposite electric 

 charges cling to the atoms. But I don't suppose that 

 other molecular forces are excluded, working directly 

 from atom to atom. Several of our leading chemists 

 have begun lately to distinguish two classes of com- 

 pounds, molecular aggregates and typical compounds. 

 The latter are united by atomic affinities, the former not. 

 Electrolytes belong to the latter class. 



If we conclude from the facts that every unit of affinity 

 of every atom is charged always with one equivalent either 

 of positive or of negative electricity, they can form com- 

 pounds, being electrically neutral, only if every unit 

 charged positively unites under the influence of a mighty 

 electric attraction with another unit charged negatively. 

 You see that this ought to produce compounds in which 

 every unit of affinity of every atom is connected with one 

 and only with one other unit of another atom. This is, 

 as you will see immediately, indeed, the modern chemical 

 theory of cjuantivalence, comprising all the saturated 

 compounds. The fact that even elementary substances, 

 with fev/ e.xceptions, have molecules composed of two 

 atoms, makes it probable that even in these cases electric 

 neutralisation is pioduced by the combination of two 

 atoms, each charged with its electric equivalent, not by 

 neutralisation of every single unit of affinity. 



But I abstain from entering into mere specialities, as, 

 for instance, the question of unsaturated compounds ; 

 perhaps I have gone already too far. I would not have 

 dared to do it if I did not feel m)self sheltered by the 



