A History of the Theories of Aether and Electricity (1910) — part 13 of 29

equator, a current would steadily flow through it. Experiments confirmatory of these views were made by Faraday himself ;* but they do not strictly prove his hypothesis that the lines of force remain at rest ; for it is easily seenf that, if they were to rotate, that part of the electromotive force which would be produced by. their rotation would be derivable from a potential, and so would produce no effect in closed circuits such as Faraday used.

Three years after the commencement of Faraday's researches on induced currents he was led to an important extension of them by an observation which was communicated to him by another worker. William Jenkin had noticed that an electric shock may be obtained with no more powerful source of electricity than a single cell, provided the wire through which the current passes is long and coiled ; the shock being felt when contact is broken. J As Jenkin did not choose to investigate the matter further, Faraday took it up, and showed§ that the powerful momentary current, which was observed when the circuit was interrupted, was really an induced current governed by the same laws as all other induced currents, but with this peculiarity, that the induced and inducing currents now flowed in the same circuit. In fact, the current in its steady state establishes in the surrounding region a magnetic field, whose lines of force are linked with the circuit ; and the removal of these lines of force when the circuit is broken originates an induced current, which greatly reinforces the primary current just before its final extinction. To this phenomenon the name of self-induction has been given.

The circumstances attending the discovery of self-induction

•Exp. Res., §$ 218, 3109, &c.

t Cf. W. Weber, Ann. d. Phys. lii (1841) ; S. Tolver Preston, Phil. Mag. xix (1885), p. 131. In 1891 S. T. Preston, Phil. Mag. xxxi, p. 100, designed a crucial experiment to test the question ; but it was not tried for want of a sufficiently delicate electrometer.

% A similar observation had been made by Henry, and published in the Amer. Jour. Sci. xxii (1832), p. 408. The spark at the rupture of a spirally-wound circuit had been often observed, e.g., by Pouillet and Nobili.

§ Exp. Res., § 1048.

O

1 94 Faraday.

occasioned a comment from Faraday on the number of sugges- tions which were continually being laid before him. He re- marked that although at different times a large number of authors had presented him with their ideas, this case of Jenkin was the only one in which any result had followed. " The volunteers are serious embarrassments generally to the experienced philosopher."*

The discoveries of Oersted, Ampere, and Faraday had shown the close connexion of magnetic with electric science. But the connexion of the different branches of electric science with each other was still not altogether clear. Although Wollaston's experiments of 1801 had in effect proved the identity in kind of the currents derived from frictional and voltaic sources, the question was still regarded as open thirty years afterwards,f no satisfactory explanation being forthcoming of the fact that frictional electricity appeared to be a surface-phenomenon, whereas voltaic electricity was conducted within the interior substance of bodies. To this question Faraday now applied him- self; and in 1833 he succeeded* in showing that every known effect of electricity — physiological, magnetic, luminous, calorific, chemical, and mechanical — may be obtained indifferently either with the electricity which is obtained by friction or with that obtained from a voltaic battery. Henceforth the identity of the two was beyond dispute.

Some misapprehension, however, has existed among later writers as to the conclusions which may be drawn from this identification. What Faraday proved is that the process which goes on in a wire connecting the terminals of a voltaic cell is of the same nature as the process which for a short time goes on in a wire by which a condenser is discharged. He did not prove,

• Bence Jones's Life of Faraday, ii, p. 45.

t Cf. John Davy, Phil. Trans., 1832, p. 259 ; W. Ritchie, ibid., p. 279. Davy suggested that the electrical power, " according to the analogy of the solar ray," might be " not a simple power, but a combination of powers, which may occur variously associated, and produce all the varieties of electricity with which we are acquainted."

J Exp. Jies.9 Series iii.

Faraday. 195

and did not profess to have proved, that this process consists in the actual movement of a quasi-substance, electricity, from one plate of the condenser to the other, or of two quasi-substances, the resinous and vitreous electricities, in opposite directions. The process had been pictured in this way by many of his predecessors, notably by Volta; and it has since been so pictured by most of his successors : but from such assumptions Faraday himself carefully abstained.

What is common to all theories, and is universally conceded, is that the rate of increase in the total quantity of electrostatic charge within any volume-element is equal to the excess of the influx over the efflux of current from it. This statement may be represented by the equation

|+divi = 0, (1)

where p denotes the volume-density of electrostatic charge, and i the current, at the place (x, y, z) at the time t. Volta's assumption is really one way of interpreting this equation physically: it presents itself when we compare equation (1) with the equation

which is the equation of continuity for a fluid of density p and velocity v : we may identify the two equations by supposing i to be of the same physical nature as the product /»v; and this is precisely what is done by those who accept Volta's assumption.

But other assumptions might be made which would equally well furnish physical interpretations to equation (1). For instance, if we suppose p to be the convergence of any vector of which i is the time-flux,* equation (1) is satisfied automatically ;

• In symbols,

div 8 = - ,

where s denotes the vector in question.

02

196 Faraday.

we can picture this vector as being of the nature of a displace- ment. By such an assumption we should avoid altogether the necessity for regarding the conduction-current as an actual flow of electric charges, or for speculating whether the drifting charges are positive or negative ; and there would be no longer anything surprising in the production of a null effect by the coalescence of electric charges of opposite signs.

Faraday himself wished to leave the matter open, and to avoid any definite assumption.* Perhaps the best indication of his views is afforded by a laboratory notej- of date 1837 : —

"After much consideration of the manner in which the electric forces are arranged in the various phenomena generally,. I have come to certain conclusions which I will endeavour to note down without committing myself to any opinion as to the cause of electricity, i.e., as to the nature of the power. If electricity exist independently of matter, then I think that the hypothesis of one fluid will not stand against that of two fluids. There are, I think, evidently what I may call two elements of power, of equal force and acting toward each other. But these powers may be distinguished only by direction, and may be no more separate than the north and south forces in the elements of a magnetic needle. They may be the polar points of the forces originally placed in the particles of matter."

It may be remarked that since the rise of the mathematical theory of electrostatics, the controversy between the supporters of the one-fluid and the two-fluid theories had become manifestly barren. The analytical equations, in which interest was now largely centred, could be interpreted equally well on either hypothesis; and there seemed to be little prospect of discriminating between them by any new experi- mental discovery. But a problem does not lose its fascination

*"His principal aim," said Helmholtz in the Faraday Lecture of 1881, " was to express in his new conceptions only facts, with the least possible use of hypothetical substances and forces. This was really a progress in general scientific method, destined to purify science from the last remains of meta- physics."

t Bence Jones's Life of Foradny^ ii, p. 77.

Faraday. 197

because it appears insoluble. " I said once to Faraday," wrote Stokes to his father-in-law in 1879, " as I sat beside him at a British Association dinner, that I thought a great step would be made when we should be able to say of electricity that which we say of light, in saying that it consists of undula- tions. He said to me he thought we were a long way off that

yet."*

For his next series of researches,! Faraday reverted to subjects which had been among the first to attract him as an apprentice attending Davy's lectures : the voltaic pile, and the relations of electricity to chemistry.

It was at this time generally supposed that the decomposi- tion of a solution, through which an electric current is passed, is due primarily to attractive and repellent forces exercised on its molecules by the metallic terminals at which the current enters and leaves the solution. Such forces had been assumed both in the hypothesis of Grothuss and Davy, and in the rival hypothesis of De La Eive ;+ the chief difference between these being that whereas Grothuss and Davy supposed a chain 'of decompositions and recompositions in the liquid, De La Rive supposed the molecules adjacent to the terminals to be the only ones decomposed, and attributed to their fragments the power of travelling through the liquid from one terminal to the other.

To test this doctrine of the influence of terminals, Faraday moistened a piece of paper in a saline solution, and supported it in the air on wax, so as to occupy part of the interval between two needle-points which were connected with an electric machine. When the machine was worked, the current was conveyed between the needle-points by way of the moistened paper and the two air-intervals on either side of it ; and under these circumstances it was found that the salt under- went decomposition. Since in this case no metallic terminals of .any kind were in contact with the solution, it was evident that

• Stokes's Scientific Correspondence, vol. i, p. 353.

t Exp. Res., § 450 (1833). £ Cf. pp. 78-9.

198 Faraday.

all hypotheses which attributed decomposition to the action of the terminals were untenable.

The ground being thus cleared by the demolition of previous theories, Faraday was at liberty to construct a theory of his own. He retained one of the ideas of Grothuss' and Davy's doctrine, namely, that a chain of decompositions and recombi- nations takes place in the liquid ; but these molecular processes he attributed not to any action of the terminals, but to a power possessed by the electric current itself, at all places in its course through the solution. If as an example we consider neighbouring molecules A, B, C, D, . . . of the compound — say water, which was at that time believed to be directly decom- posed by the current — Faraday supposed that before the passage of the current the hydrogen of A would be in close union with the oxygen of A, and also in a less close relation with the oxygen atoms of B, C, D, . . . : these latter relations being conjectured to be the cause of the attraction of aggregation in solids and fluids.* When an electric current is sent through the liquid, the affinity of the hydrogen of A for the oxygen of B is strengthened, if A and B lie along the direction of the current ; while the hydrogen of A withdraws some of its bonds from the oxygen of A, with which it is at the moment combined. So long as the hydrogen and oxygen of A remain in association, the state thus induced is merely one of polarization ; but the compound molecule is unable to stand the strain thus imposed on it, and the hydrogen and oxygen of A part company from each other. Thus decompositions take place, followed by recombinations : with the result that after each exchange an oxygen atom associates itself with a partner nearer to the positive terminal, while a hydrogen atom associates with a partner nearer to the negative terminal.

This theory explains why, in all ordinary cases, the evolved substances appear only at the terminals ; for the terminals are the limiting surfaces of the decomposing substance ; and, except at them, every particle finds other particles having a contrary

*Exp. lies., §523.

Faraday. 199

tendency with which it can combine. It also explains why, in numerous cases, the atoms of the evolved substances are not retained by the terminals (an obvious difficulty in the way of all theories which suppose the terminals to attract the atoms) : for the evolved substances v are expelled from the liquid, not drawn out by an attraction.

Many of the perplexities which had harassed the older theories were at once removed when the phenomena were re- garded from Faraday's point of view. Thus, mere mixtures (as opposed to chemical compounds) are not separated into their constituents by the electric current ; although there would seem to be no reason why the Grothuss-Davy polar attraction should not operate as well on elements contained in mixtures as on elements contained in compounds.

In the latter part of the same year (1833) Faraday took up the subject again.* It was at this time that he introduced the terms which have ever since been generally used to describe the phenomena of electro-chemical decomposition. To the terminals by which the electric current passes into or out of the decomposing body he gave the name electrodes. The electrode of high potential, at which oxygen, chlorine, acids, &c., are evolved, he called the anode, and the electrode of low potential, at which metals, alkalis, and bases are evolved, the cathode. Those bodies which are decomposed directly by the current he named electrolytes ; the parts into which they are decomposed, ions ; the acid ions, which travel to the anode, he named anions ; and the metallic ions, which pass to the cathode, cations.

Faraday now proceeded to test the truth of a supposition which he had published rather more than a year previously ,f and which indeed had apparently been suspected by Gay-Lussac and ThenardJ so early as 1811; namely, that the rate at which an electrolyte is decomposed depends solely on the intensity of the electric current passing through it, and not at all on the size of the electrodes or the strength of the solution. Having

• Exp. Res., § 661. f /'., § 377 (Dec. 1832).

£ Recherches physico-chimiqucs faites sur la pile ; Paris, 1811, p. 12.

200 Faraday.

established the accuracy of this law,* he found by a comparison of different electrolytes that the mass of any ion liberated by a given quantity of electricity is proportional to its chemical equivalent, i.e. to the amount of it required to combine with some standard mass of some standard element. If an element is %-valent, so that one of its atoms can hold in combination n atoms of hydrogen, the chemical equivalent of this element may be taken to be 1/n of its atomic weight ; and therefore Faraday's result may be expressed by saying that an electric current will liberate exactly one atom of the element in question in the time which it would take to liberate n atoms of hydrogen.-)-

The quantitative law seemed to Faraday:}: to indicate that " the atoms of matter are in some way endowed or associated with electrical powers, to which they owe their most striking qualities, and amongst them their mutual chemical affinity." Looking at the facts of electrolytic decomposition from this point of view, he showed how natural it is to suppose that the electricity which passes through the electrolyte is the exact equivalent of that which is possessed by the atoms separated at the electrodes ; which implies that there is a certain absolute quantity of the electric power associated with each atom of matter.

The claims of this splendid speculation he advocated with conviction. " The harmony," he wrote, § " which it introduces into the associated theories of definite proportions and electro- chemical affinity is very great. According to it, the equivalent weights of bodies are simply those quantities of them which contain equal quantities of electricity, or have naturally equal electric powers ; it being the ELECTRICITY which determines the equivalent number, because it determines the combining force. Or, if we adopt the atomic theory or phraseology, then the

*Exp. Res., §§ 713-821.

t In the modern units, 96580 coulombs of electricity must pass round the circuit in order to liberate of each ion a number of grams equal to the quotient of the atomic weight by the valency.

J Exp. Res., § 852. § Ibid., § 869.

Faraday. 201

atoms of bodies which are equivalent to each other in their ordinary chemical action, have equal quantities of electricity naturally associated with them. " But," he added, " I must confess I am jealous of the term atom : for though it is very easy to talk of atoms, it is very difficult to form a clear idea of their nature, especially when compound bodies are under consideration."

These discoveries and ideas tended to confirm Faraday in preferring, among the rival theories of the voltaic cell, that one to which all his antecedents and connexions predisposed him. The controversy between the supporters of Volta's contact hypothesis on the one hand, and the chemical hypothesis of Davy and Wollaston on the other, had now been carried on for a generation without any very decisive result. In Germany and Italy the contact explanation was generally accepted, under the influence of Christian Heiririch Pfaff, of Kiel (b. 1773, d. 1852), and of Ohm, and, among the younger men, of Gustav Theodor Fechner (b. 1801, d. 1887), of Leipzig,* and Stefano Marianini (b. 1790, d. 1866), of Modena. Among French writers De La Eive, of Geneva, was, as we have seen, active in support of the chemical hypothesis; and this side in the dispute had always been favoured by the English philosophers.

There is no doubt that when two different metals are put in contact, a difference of potential is set up between them without any apparent chemical action ; but while the contact party regarded this as a direct manifestation of a "contact- force " distinct in kind from all other known forces of nature,

• Johaim Christian Poggendorff (b. 1796, d. 1877), of Berlin, for long the editor of the Annalen der Physik, leaned originally to the chemical side, but in 1838 became convinced of the truth of the contact theory, which he afterwards actively defended. Moritz Hermann Jacobi (b. 1801, d. 1874), of Dorpat, is also to be mentioned among its advocates.

Faraday's first series of investigations on this subject were made in 1834 : Exp, £es., series viii. In 1836 De La Kive followed on the same side with his Eccherches sur la Cause de V Electr. Voltaique. The views of Faraday and De La Rive were criticized by Pfaff, Revision der Lehre vom Galvanistntts, Kiel, 1837, and by Fechner, Ann. d. Phys., xlii (1837), p. 481, and xliii (1838), p. 433 : translated Phil. Mag., xiii (1838), pp. 205, 367. Faraday returned to the question in 1840, Exp, Jtes., series xvi and xvii.

202 Faraday.

the chemical party explained it as a consequence of chemical affinity or incipient chemical action between the metals and the surrounding air or moisture. There is also no doubt that the continued activity of a voltaic cell is always accompanied by chemical unions or decompositions ; but while the chemical party asserted that these constitute the efficient source of the- current, the contact party regarded them as secondary actions, and attributed the continual circulation of electricity to the perpetual tendency of the electromotive force of contact to transfer charge from one substance to another.

One of the most active supporters of the chemical theory among the English physicists immediately preceding Faraday was Peter Mark Eoget (b. 1779, d. 1869), to whom are due two- of the strongest arguments in its favour. In the first place, carefully distinguishing between the quantity of electricity put into circulation by a cell and the tension at which this electricity is furnished, he showed that the latter quantity depends on the " energy of the chemical action "* — a fact which, when taken together with Faraday's discovery that the quantity of electricity put into circulation depends on the amount of chemicals con- sumed, places the origin of voltaic activity beyond all question. Koget's principle was afterwards verified by Faradayf and by De La EiveJ; " the electricity of the voltaic pile is proportionate in its intensity to the intensity of the affinities concerned in its production," said the former in 1834; while De La Kive wrote in 1836, " The intensity of the currents developed in combinations and in decompositions is exactly proportional to the degree of affinity which subsists between the atoms whose combination or separation has given rise to these currents."

• " The absolute quantity of electricity which is thus developed, and made to circulate, will depend upon a variety of circumstances, such as the extent of the surfaces in chemical action, the facilities afforded to its transmission, &c. But its degree of intensity, or tension, as it is often termed, will be regulated by other causes, and more especially by the energy of the chemical action." Roget's Galvanism (1832), § 70.

t Exp. Res., §§ 908, 909, 916, 988, 1958.

I Annales de Chim., Ixi (1836), p. 38.

Faraday. 203

Not resting here, however, Koget brought up another argu- ment of far-reaching significance. " If," he wrote,* " there could exist a power having the property ascribed to it by the [contact] hypothesis, namely, that of giving continual impulse to a fluid in one constant direction, without being exhausted by its own action, it would differ essentially from all the other known powers in nature. All the powers and sources of motion, with the operation of which we are acquainted, when producing their peculiar effects, are expended in the same proportion as those effects are produced ; and hence arises the impossibility of obtaining by their agency a perpetual effect ; or, in other words, a perpetual motion. But the electro-motive force ascribed by Yolta to the metals when in contact is a force which, as long as a free course is allowed to the electricity it sets in motion, is never expended, and continues to be exerted with undi- minished power, in the production of a never-ceasing effect. Against the truth of such a supposition the probabilities are all but infinite."

This principle, which is little less than the doctrine of conservation of energy applied to a voltaic cell, was reasserted by Faraday. The process imagined by the contact school " would," he wrote, "indeed be a creation of 'power -, like no other force in nature." In all known cases energy is not generated, but only transformed. There is no such thing in the world as "a pure creation of force; a production of power without a corresponding exhaustion of something to supply it."f

As time went on, each of the rival theories of the cell became modified in the direction of the other. The contact party admitted the importance of the surfaces at which the metals are in contact with the liquid, where of course the chief chemical action takes place ; and the chemical party confessed their inability to explain the state of tension which subsists before the circuit is closed, without introducing hypotheses just as uncertain as that of contact force.

*Roget's Galvanism (1832), § 113. •t Exp.Res., § 2071 (1840).

204 Faraday.

Faraday's own view on this point* was that a plate of amalgamated zinc, when placed in dilute sulphuric acid, " has power so far to act, by its attraction for the oxygen of the particles'in contact with it, as to place the similar forces already active between these and the other particles of oxygen and the particles of hydrogen in the water, in a peculiar state of tension or polarity, and probably also at the same time to throw those of its own particles which are in contact with the water into a similar but opposed state. Whilst this state is retained, no further change occurs: but when it is relieved by completion of the circuit, in which case the forces determined in opposite directions, with respect to the zinc and the electro- lyte, are found exactly competent to neutralize each other, then a series of decompositions and recompositions takes place amongst the particles of oxygen and hydrogen which constitute the water, between the place of contact with the platina and the place where the zinc is active : these intervening particles being evidently in close dependence upon and relation to each other. The zinc forms a direct compound with those particles of oxygen which were, previously, in divided relation to both it and the hydrogen : the oxide is removed by the acid, and a fresh surface of zinc is presented to the water, to renew and repeat the action."

These ideas were developed further by the later adherents of the chemical theory, especially by Faraday's friend Christian Friedrich Schonbein,f of Basle (6. 1799, d. 1868), the discoverer of ozone. Schonbein made the hypothesis more definite by assuming that when the circuit is open, the molecules of water adjacent to the zinc plate are electrically polarized, the oxygen side of each molecule being turned towards the zinc and being negatively charged, while the hydrogen side is turned away from the zinc and is positively charged. In the third quarter

• Exp. &»., § 949.

t Ann. d. Phys., Ixxviii (1849), p. 289, translated Archives des sc. phys., xiii (1850), p. 192. Faraday and Schonbein for many years carried on a correspondence, which has been edited by G. W. A. Kahlbaum and F. V. Darbishire : London, Williams and Norgate.

Faraday. 205

of the nineteenth century, the general opinion was in favour of some such conception as this. Helmholtz* attempted to- grasp the molecular processes more intimately by assuming that the different chemical elements have different attractive- powers (exerted only at small distances) for the vitreous and resinous electricities : thus potassium and zinc have strong attractions for positive charges, while oxygen, chlorine, and bromine have strong attractions for negative electricity. This differs from Volta's original hypothesis in little else but in assuming two electric fluids where Volta assumed only one. It is evident that the contact difference of potential; between two metals may be at once explained by Helmholtz's, hypothesis, as it was by Volta's ; and the activity of the voltaic cell may be referred to the same principles : for the two ions of which the liquid molecules are composed will also possess different attractive powers for the electricities, and may be supposed to be united respectively with vitreous and resinous, charges. Thus when two metals are immersed in the liquid,^ the circuit being open, the positive ions are attracted to the negative metal and the negative ions to the positive metal,, thereby causing a polarized arrangement of the liquid molecules near the metals. When the circuit is closed, the positively charged surface of the positive metal is dissolved into the fluid;, and as the atoms carry their charge with them, the positive charge on the immersed surface of this metal must be per- petually renewed by a current flowing in the outer circuit.

It will be seen that Helmholtz did not adhere to Davy'ss doctrine of the electrical nature of chemical affinity quite as, simply or closely as Faraday, who preferred it in its most direct and uncompromising form. " All the facts show us," he wrote,f "that that power commonly called chemical affinity can be> communicated to a distance through the metals and certain forms of carbon ; that the electric current is only another form of the forces of chemical affinity ; that its power is in proportion.

• In his celebrated memoir of 1847 on the Conservation, .o£.Huergy. t Exp. Ties., § 918.

206 Faraday.

to the chemical affinities producing it ; that when it is deficient in force it may be helped by calling in chemical aid, the want in the former being made up by an equivalent of the latter; that, in other words, the forces termed chemical affinity and electricity are one and the same."

In the interval between Faraday's earlier and later papers on the cell, some important results on the same subject were published by Frederic Daniell (b. 1790, d. 1845), Professor of Chemistry in King's College, London.* Daniell showed that when a current is passed through a solution of a salt in water, the ions which carry the current are those derived from the salt, and not the oxygen and hydrogen ions derived from the water ; this follows since a current divides itself between different mixed electrolytes according to the difficulty of decomposing each, and it is known that pure water can be electrolysed only with great difficulty. Daniell further showed that the ions arising from (say) sodium sulphate are not represented by Na20 and S03, but by Na and S04 ; and that in such a case as this, sulphuric acid is formed at the anode and soda at the cathode by secondary action, giving rise to the observed evolution of oxygen and hydrogen respectively at these terminals.

The researches of Faraday on the decomposition of chemical compounds placed between electrodes maintained at different potentials led him in 1837 to reflect on the behaviour of such substances as oil of turpentine or sulphur, when placed in the same situation. These bodies do not conduct electricity, and are not decomposed ; but if the metallic faces of a condenser are maintained at a definite potential difference, and if the space between them is occupied by one of these insulating substances, it is found that the charge on either face depends on the nature of the insulating substance. If for any particular insulator the charge has a value s times the value which it would have if the intervening body were air, the number f may be regarded as a measure of the influence which the insulator exerts on the propagation of electrostatic action

• Phil. Trans., 1839, p. 97.

Faraday. 207

through it : it was called by Faraday the specific inductive •capacity of the insulator.*

The discovery of this property of insulating substances or dielectrics raised the question as to whether it could be harmonized with the old ideas of electrostatic action. Consider, for example, the force of attraction or repulsion between two small electrically- charged bodies. So long as they are in air, the force is proportional to the inverse square of the distance ; but if the medium in which they are immersed be partly changed — e.g., if a globe of sulphur be inserted in the intervening space — this law is no longer valid : the change in the dielectric affects the distribution of electric intensity throughout the •entire field.

The problem could be satisfactorily solved only by forming a physical conception of the action of dielectrics : and such a conception Faraday now put forward.

The original idea had been promulgated long before by his master Davy. Davy, it will be remembered,f in his explanation of the voltaic pile, had supposed that at first, before chemical decompositions take place, the liquid plays a part analogous to that of the glass in a Leyden jar, and that in this is involved an electric polarization of the liquid molecules.^ This hypothesis was now developed by Faraday. Keferring first to his own work on electrolysis, he asserted§ that the behaviour of a dielectric is exactly the same as that of an electrolyte, up to the point at which the electrolyte breaks down under the electric stress ; a dielectric being, in fact, a body which is capable of sustaining the stress without suffering decomposition.

" For," he argued,|| " let the electrolyte be water, a plate of ice being coated with platina foil on its two surfaces, and these

• Exp. Res., § 1252 (1837). Cavendish had discovered specific inductive capacity long before, but his papers were still unpublished.

t Cf. p. 77.

\ This is expressly stated in Davy's Elements of Chemical Philosophy (1812), Div. i, § 7, where he lays it down that an essential " property of non-conductors" is "to receive electrical polarities."

$ Exp. Res., §§ 1164, 1338, 1343, 1621.

|| Exp. Res., § 1164.

208 Faraday.

coatings connected with any continued source of the two electrical powers, the ice will charge like a Leyden arrangement, presenting a case of common induction, but no current will pass. If the ice be liquefied, the induction will now fall to a certain degree, because a current can now pass ; but its passing is dependent upon a peculiar molecular arrangement of the particles consistent with the transfer of the elements of the electrolyte in opposite directions . . . As, therefore, in the electrolytic action, induction appeared to bethejfe£ step,and decomposition the second (the power of separating these steps from each other by giving the solid or fluid condition to the electrolyte being in our hands) ;: as the induction was the same in its nature as that through air, glass, wax, &c., produced by any of the ordinary means ; and as the whole effect in the electrolyte appeared to be an action of the particles thrown into a peculiar or polarized state, I was glad to suspect that common induction itself was in all cases an action of contiguous particles, and that electrical action at a distance (i.e., ordinary inductive action) never occurred except through the influence of the intervening matter."

Thus at the root of Faraday's conception of electrostatic induction lay this idea that the whole of the insulating medium through which the action takes place is in a state of polarization similar to that which precedes decomposition in an electrolyte. " Insulators," he wrote,* " may be said to be bodies whose particles can retain the polarized state, whilst conductors are those whose particles cannot be permanently polarized."