Faraday as a Discoverer Part 3

Litmus reveals, as you know, the action of an acid by turning red, turmeric reveals the action of an alkali by turning brown. Sulphate of soda, you know, is a salt compounded of the alkali soda and sulphuric acid. The voltaic current pa.s.sing through a solution of this salt so decomposes it, that sulphuric acid appears at one pole of the decomposing cell and alkali at the other. Faraday steeped a piece of litmus paper and a piece of turmeric paper in a solution of sulphate of soda: placing each of them upon a separate plate of gla.s.s, he connected them together by means of a string moistened with the same solution.

He then attached one of them to the positive conductor of an electric machine, and the other to the gas-pipes of this building. These he called his 'discharging train.' On turning the machine the electricity pa.s.sed from paper to paper through the string, which might be varied in length from a few inches to seventy feet without changing the result.

The first paper was reddened, declaring the presence of sulphuric acid; the second was browned, declaring the presence of the alkali soda. The dissolved salt, therefore, arranged in this fashion, was decomposed by the machine, exactly as it would have been by the voltaic current.

When instead of using the positive conductor he used the negative, the positions of the acid and alkali were reversed. Thus he satisfied himself that chemical decomposition by the machine is obedient to the laws which rule decomposition by the pile.

And now he gradually abolishes those so-called poles, to the attraction of which electric decomposition had been ascribed. He connected a piece of turmeric paper moistened with the sulphate of soda with the positive conductor of his machine; then he placed a metallic point in connection with his discharging train opposite the moist paper, so that the electricity should discharge through the air towards the point. The turning of the machine caused the corners of the piece of turmeric paper opposite to the point to turn brown, thus declaring the presence of alkali. He changed the turmeric for litmus paper, and placed it, not in connection with his conductor, but with his discharging train, a metallic point connected with the conductor being fixed at a couple of inches from the paper; on turning the machine, acid was liberated at the edges and corners of the litmus. He then placed a series of pointed pieces of paper, each separate piece being composed of two halves, one of litmus and the other of turmeric paper, and all moistened with sulphate of soda, in the line of the current from the machine. The pieces of paper were separated from each other by s.p.a.ces of air. The machine was turned; and it was always found that at the point where the electricity entered the paper, litmus was reddened, and at the point where it quitted the paper, turmeric was browned. 'Here,' he urges, 'the poles are entirely abandoned, but we have still electrochemical decomposition.' It is evident to him that instead of being attracted by the poles, the bodies separated are ejected by the current. The effects thus obtained with poles of air he also succeeded in obtaining with poles of water. The advance in Faraday's own ideas made at this time is indicated by the word 'ejected.' He afterwards reiterates this view: the evolved substances are expelled from the decomposing body, and 'not drawn out by an attraction.

Having abolished this idea of polar attraction, he proceeds to enunciate and develop a theory of his own. He refers to Davy's celebrated Bakerian Lecture, given in 1806, which he says 'is almost entirely occupied in the consideration of electrochemical decompositions.' The facts recorded in that lecture Faraday regards as of the utmost value. But 'the mode of action by which the effects take place is stated very generally; so generally, indeed, that probably a dozen precise schemes of electrochemical action might be drawn up, differing essentially from each other, yet all agreeing with the statement there given.'

It appears to me that these words might with justice be applied to Faraday's own researches at this time. They furnish us with results of permanent value; but little help can be found in the theory advanced to account for them. It would, perhaps, be more correct to say that the theory itself is hardly presentable in any tangible form to the intellect. Faraday looks, and rightly looks, into the heart of the decomposing body itself; he sees, and rightly sees, active within it the forces which produce the decomposition, and he rejects, and rightly rejects, the notion of external attraction; but beyond the hypothesis of decompositions and recompositions, enunciated and developed by Grothuss and Davy, he does not, I think, help us to any definite conception as to how the force reaches the decomposing ma.s.s and acts within it. Nor, indeed, can this be done, until we know the true physical process which underlies what we call an electric current.

Faraday conceives of that current as 'an axis of power having contrary forces exactly equal in amount in opposite directions'; but this definition, though much quoted and circulated, teaches us nothing regarding the current. An 'axis' here can only mean a direction; and what we want to be able to conceive of is, not the axis along which the power acts, but the nature and mode of action of the power itself. He objects to the vagueness of De la Rive; but the fact is, that both he and De la Rive labour under the same difficulty. Neither wishes to commit himself to the notion of a current compounded of two electricities flowing in two opposite directions: but the time had not come, nor is it yet come, for the displacement of this provisional fiction by the true mechanical conception. Still, however indistinct the theoretic notions of Faraday at this time may be, the facts which are rising before him and around him are leading him gradually, but surely, to results of incalculable importance in relation to the philosophy of the voltaic pile.

He had always some great object of research in view, but in the pursuit of it he frequently alighted on facts of collateral interest, to examine which he sometimes turned aside from his direct course. Thus we find the series of his researches on electrochemical decomposition interrupted by an inquiry into 'the power of metals and other solids, to induce the combination of gaseous bodies.' This inquiry, which was received by the Royal Society on Nov. 30, 1833, though not so important as those which precede and follow it, ill.u.s.trates throughout his strength as an experimenter. The power of spongy platinum to cause the combination of oxygen and hydrogen had been discovered by Dobereiner in 1823, and had been applied by him in the construction of his well-known philosophic lamp. It was shown subsequently by Dulong and Thenard that even a platinum wire, when perfectly cleansed, may be raised to incandescence by its action on a jet of cold hydrogen.

In his experiments on the decomposition of water, Faraday found that the positive platinum plate of the decomposing cell possessed in an extraordinary degree the power of causing oxygen and hydrogen to combine. He traced the cause of this to the perfect cleanness of the positive plate. Against it was liberated oxygen, which, with the powerful affinity of the 'nascent state,' swept away all impurity from the surface against which it was liberated. The bubbles of gas liberated on one of the platinum plates or wires of a decomposing cell are always much smaller, and they rise in much more rapid succession than those from the other. Knowing that oxygen is sixteen times heavier than hydrogen, I have more than once concluded, and, I fear, led others into the error of concluding, that the smaller and more quickly rising bubbles must belong to the lighter gas. The thing appeared so obvious that I did not give myself the trouble of looking at the battery, which would at once have told me the nature of the gas. But Faraday would never have been satisfied with a deduction if he could have reduced it to a fact. And he has taught me that the fact here is the direct reverse of what I supposed it to be. The small bubbles are oxygen, and their smallness is due to the perfect cleanness of the surface on which they are liberated. The hydrogen adhering to the other electrode swells into large bubbles, which rise in much slower succession; but when the current is reversed, the hydrogen is liberated upon the cleansed wire, and then its bubbles also become small.

Footnotes to Chapter 5

[1] Buff finds the quant.i.ty of electricity a.s.sociated with one milligramme of hydrogen in water to be equal to 45,480 charges of a Leyden jar, with a height of 480 millimetres, and a diameter of 160 millimetres. Weber and Kohlrausch have calculated that, if the quant.i.ty of electricity a.s.sociated with one milligramme of hydrogen in water were diffused over a cloud at a height of 1000 metres above the earth, it would exert upon an equal quant.i.ty of the opposite electricity at the earth's surface an attractive force of 2,268,000 kilogrammes. (Electrolytische Maasbestimmungen, 1856, p. 262.)

[2] Faraday, sa Vie et ses Travaux, p. 20.

Chapter 6.

Laws of electro-chemical decomposition.

In our conceptions and reasonings regarding the forces of nature, we perpetually make use of symbols which, when they possess a high representative value, we dignify with the name of theories. Thus, prompted by certain a.n.a.logies, we ascribe electrical phenomena to the action of a peculiar fluid, sometimes flowing, sometimes at rest. Such conceptions have their advantages and their disadvantages; they afford peaceful lodging to the intellect for a time, but they also circ.u.mscribe it, and by-and-by, when the mind has grown too large for its lodging, it often finds difficulty in breaking down the walls of what has become its prison instead of its home.[1]

No man ever felt this tyranny of symbols more deeply than Faraday, and no man was ever more a.s.siduous than he to liberate himself from them, and the terms which suggested them. Calling Dr. Whewell to his aid in 1833, he endeavoured to displace by others all terms tainted by a foregone conclusion. His paper on Electro-chemical Decomposition, received by the Royal Society on January 9, 1834, opens with the proposal of a new terminology. He would avoid the word 'current' if he could.[2] He does abandon the word 'poles' as applied to the ends of a decomposing cell, because it suggests the idea of attraction, subst.i.tuting for it the perfectly natural term Electrodes. He applied the term Electrolyte to every substance which can be decomposed by the current, and the act of decomposition he called Electrolysis. All these terms have become current in science. He called the positive electrode the Anode, and the negative one the Cathode, but these terms, though frequently used, have not enjoyed the same currency as the others. The terms Anion and Cation, which he applied to the const.i.tuents of the decomposed electrolyte, and the term Ion, which included both anions and cations, are still less frequently employed.

Faraday now pa.s.ses from terminology to research; he sees the necessity of quant.i.tative determinations, and seeks to supply himself with a measure of voltaic electricity. This he finds in the quant.i.ty of water decomposed by the current. He tests this measure in all possible ways, to a.s.sure himself that no error can arise from its employment. He places in the course of one and the same current a series of cells with electrodes of different sizes, some of them plates of platinum, others merely platinum wires, and collects the gas liberated on each distinct pair of electrodes. He finds the quant.i.ty of gas to be the same for all.

Thus he concludes that when the same quant.i.ty of electricity is caused to pa.s.s through a series of cells containing acidulated water, the electro-chemical action is independent of the size of the electrodes.[3]

He next proves that variations in intensity do not interfere with this equality of action. Whether his battery is charged with strong acid or with weak; whether it consists of five pairs or of fifty pairs; in short, whatever be its source, when the same current is sent through his series of cells the same amount of decomposition takes place in all. He next a.s.sures himself that the strength or weakness of his dilute acid does not interfere with this law. Sending the same current through a series of cells containing mixtures of sulphuric acid and water of different strengths, he finds, however the proportion of acid to water might vary, the same amount of gas to be collected in all the cells.

A crowd of facts of this character forced upon Faraday's mind the conclusion that the amount of electro-chemical decomposition depends, not upon the size of the electrodes, not upon the intensity of the current, not upon the strength of the solution, but solely upon the quant.i.ty of electricity which pa.s.ses through the cell. The quant.i.ty of electricity he concludes is proportional to the amount of chemical action. On this law Faraday based the construction of his celebrated Voltameter, or Measure of Voltaic electricity.

But before he can apply this measure he must clear his ground of numerous possible sources of error. The decomposition of his acidulated water is certainly a direct result of the current; but as the varied and important researches of MM. Becquerel, De la Rive, and others had shown, there are also secondary actions which may materially interfere with and complicate the pure action of the current. These actions may occur in two ways: either the liberated ion may seize upon the electrode against which it is set free, forming a chemical compound with that electrode; or it may seize upon the substance of the electrolyte itself, and thus introduce into the circuit chemical actions over and above those due to the current. Faraday subjected these secondary actions to an exhaustive examination. Instructed by his experiments, and rendered competent by them to distinguish between primary and secondary results, he proceeds to establish the doctrine of 'Definite Electro-chemical Decomposition.'

Into the same circuit he introduced his voltameter, which consisted of a graduated tube filled with acidulated water and provided with platinum plates for the decomposition of the water, and also a cell containing chloride of tin. Experiments already referred to had taught him that this substance, though an insulator when solid, is a conductor when fused, the pa.s.sage of the current being always accompanied by the decomposition of the chloride. He wished to ascertain what relation this decomposition bore to that of the water in his voltameter.

Completing his circuit, he permitted the current to continue until 'a reasonable quant.i.ty of gas' was collected in the voltameter. The circuit was then broken, and the quant.i.ty of tin liberated compared with the quant.i.ty of gas. The weight of the former was 3.2 grains, that of the latter 0.49742 of a grain. Oxygen, as you know, unites with hydrogen in the proportion of 8 to 1, to form water. Calling the equivalent, or as it is sometimes called, the atomic weight of hydrogen 1, that of oxygen is 8; that of water is consequently 8 + 1 or 9. Now if the quant.i.ty of water decomposed in Faraday's experiment be represented by the number 9, or in other words by the equivalent of water, then the quant.i.ty of tin liberated from the fused chloride is found by an easy calculation to be 57.9, which is almost exactly the chemical equivalent of tin. Thus both the water and the chloride were broken up in proportions expressed by their respective equivalents. The amount of electric force which wrenched asunder the const.i.tuents of the molecule of water was competent, and neither more nor less than competent, to wrench asunder the const.i.tuents of the molecules of the chloride of tin. The fact is typical. With the indications of his voltameter he compared the decompositions of other substances, both singly and in series. He submitted his conclusions to numberless tests. He purposely introduced secondary actions. He endeavoured to hamper the fulfilment of those laws which it was the intense desire of his mind to see established. But from all these difficulties emerged the golden truth, that under every variety of circ.u.mstances the decompositions of the voltaic current are as definite in their character as those chemical combinations which gave birth to the atomic theory. This law of Electro-chemical Decomposition ranks, in point of importance, with that of Definite Combining Proportions in chemistry.

Footnotes to Chapter 6

[1] I copy these words from the printed abstract of a Friday evening lecture, given by myself, because they remind me of Faraday's voice, responding to the utterance by an emphatic 'hear! hear!'--Proceedings of the Royal Inst.i.tution, vol.

ii. p. 132.

[2] In 1838 he expresses himself thus:--'The word current is so expressive in common language that when applied in the consideration of electrical phenomena, we can hardly divest it sufficiently of its meaning, or prevent our minds from being prejudiced by it.'--Exp. Resear., vol. i. p. 515. ($ 1617.)

[3] This conclusion needs qualification. Faraday overlooked the part played by ozone.

Chapter 7.

Origin of power in the voltaic pile.

In one of the public areas of the town of Como stands a statue with no inscription on its pedestal, save that of a single name, 'Volta.' The bearer of that name occupies a place for ever memorable in the history of science. To him we owe the discovery of the voltaic pile, to which for a brief interval we must now turn our attention.

The objects of scientific thought being the pa.s.sionless laws and phenomena of external nature, one might suppose that their investigation and discussion would be completely withdrawn from the region of the feelings, and pursued by the cold dry light of the intellect alone.

This, however, is not always the case. Man carries his heart with him into all his works. You cannot separate the moral and emotional from the intellectual; and thus it is that the discussion of a point of science may rise to the heat of a battle-field. The fight between the rival optical theories of Emission and Undulation was of this fierce character; and scarcely less fierce for many years was the contest as to the origin and maintenance of the power of the voltaic pile. Volta himself supposed it to reside in the Contact of different metals.

Here was exerted his 'Electro-motive force,' which tore the combined electricities asunder and drove them as currents in opposite directions.

To render the circulation of the current possible, it was necessary to connect the metals by a moist conductor; for when any two metals were connected by a third, their relation to each other was such that a complete neutralisation of the electric motion was the result. Volta's theory of metallic contact was so clear, so beautiful, and apparently so complete, that the best intellects of Europe accepted it as the expression of natural law.

Volta himself knew nothing of the chemical phenomena of the pile; but as soon as these became known, suggestions and intimations appeared that chemical action, and not metallic contact, might be the real source of voltaic electricity. This idea was expressed by Fabroni in Italy, and by Wollaston in England. It was developed and maintained by those 'admirable electricians,' Becquerel, of Paris, and De la Rive, of Geneva. The Contact Theory, on the other hand, received its chief development and ill.u.s.tration in Germany. It was long the scientific creed of the great chemists and natural philosophers of that country, and to the present hour there may be some of them unable to liberate themselves from the fascination of their first-love.

After the researches which I have endeavoured to place before you, it was impossible for Faraday to avoid taking a side in this controversy.

He did so in a paper 'On the Electricity of the Voltaic Pile,' received by the Royal Society on the 7th of April, 1834. His position in the controversy might have been predicted. He saw chemical effects going hand in hand with electrical effects, the one being proportional to the other; and, in the paper now before us, he proved that when the former was excluded, the latter were sought for in vain. He produced a current without metallic contact; he discovered liquids which, though competent to transmit the feeblest currents--competent therefore to allow the electricity of contact to flow through them if it were able to form a current--were absolutely powerless when chemically inactive.

One of the very few experimental mistakes of Faraday occurred in this investigation. He thought that with a single voltaic cell he had obtained the spark before the metals touched, but he subsequently discovered his error. To enable the voltaic spark to pa.s.s through air before the terminals of the battery were united, it was necessary to exalt the electro-motive force of the battery by multiplying its elements; but all the elements Faraday possessed were unequal to the task of urging the spark across the shortest measurable s.p.a.ce of air.

Nor, indeed, could the action of the battery, the different metals of which were in contact with each other, decide the point in question.

Still, as regards the ident.i.ty of electricities from various sources, it was at that day of great importance to determine whether or not the voltaic current could jump, as a spark, across an interval before contact. Faraday's friend, Mr. Ga.s.siot, solved this problem. He erected a battery of 4000 cells, and with it urged a stream of sparks from terminal to terminal, when separated from each other by a measurable s.p.a.ce of air.

The memoir on the 'Electricity of the Voltaic Pile,' published in 1834, appears to have produced but little impression upon the supporters of the contact theory. These indeed were men of too great intellectual weight and insight lightly to take up, or lightly to abandon a theory.

Faraday therefore resumed the attack in a paper, communicated to the Royal Society on the 6th of February, 1840. In this paper he hampered his antagonists by a crowd of adverse experiments. He hung difficulty after difficulty about the neck of the contact theory, until in its efforts to escape from his a.s.saults it so changed its character as to become a thing totally different from the theory proposed by Volta. The more persistently it was defended, however, the more clearly did it show itself to be a congeries of devices, bearing the stamp of dialectic skill rather than of natural truth.

In conclusion, Faraday brought to bear upon it an argument which, had its full weight and purport been understood at the time, would have instantly decided the controversy. 'The contact theory,' he urged, 'a.s.sumed that a force which is able to overcome powerful resistance, as for instance that of the conductors, good or bad, through which the current pa.s.ses, and that again of the electrolytic action where bodies are decomposed by it, can arise out of nothing; that, without any change in the acting matter, or the consumption of any generating force, a current shall be produced which shall go on for ever against a constant resistance, or only be stopped, as in the voltaic trough, by the ruins which its exertion has heaped up in its own course. This would indeed be a creation of power, and is like no other force in nature. We have many processes by which the form of the power may be so changed, that an apparent conversion of one into the other takes place. So we can change chemical force into the electric current, or the current into chemical force. The beautiful experiments of Seebeck and Peltier show the convertibility of heat and electricity; and others by Oersted and myself show the convertibility of electricity and magnetism. But in no case, not even in those of the Gymnotus and Torpedo, is there a pure creation or a production of power without a corresponding exhaustion of something to supply it.'

These words were published more than two years before either Mayer printed his brief but celebrated essay on the Forces of Inorganic Nature, or Mr. Joule published his first famous experiments on the Mechanical Value of Heat. They ill.u.s.trate the fact that before any great scientific principle receives distinct enunciation by individuals, it dwells more or less clearly in the general scientific mind. The intellectual plateau is already high, and our discoverers are those who, like peaks above the plateau, rise a little above the general level of thought at the time.

But many years prior even to the foregoing utterance of Faraday, a similar argument had been employed. I quote here with equal pleasure and admiration the following pa.s.sage written by Dr. Roget so far back as 1829. Speaking of the contact theory, he says:--'If there could exist a power having the property ascribed to it by the 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 known powers in nature. All the powers and sources of motion with the operation of which we are acquainted, when producing these 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 Volta 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 excited with undiminished power in the production of a never-ceasing effect. Against the truth of such a supposition the probabilities are all but infinite.' When this argument, which he employed independently, had clearly fixed itself in his mind, Faraday never cared to experiment further on the source of electricity in the voltaic pile. The argument appeared to him 'to remove the foundation itself of the contact theory,'

and he afterwards let it crumble down in peace.[1]

Footnote to Chapter 7

[1] To account for the electric current, which was really the core of the whole discussion, Faraday demonstrated the impotence of the Contact Theory as then enunciated and defended. Still, it is certain that two different metals, when brought into contact, charge themselves, the one with positive and the other with negative electricity. I had the pleasure of going over this ground with Kohlrausch in 1849, and his experiments left no doubt upon my mind that the contact electricity of Volta was a reality, though it could produce no current. With one of the beautiful instruments devised by himself, Sir William Thomson has rendered this point capable of sure and easy demonstration; and he and others now hold what may be called a contact theory, which, while it takes into account the action of the metals, also embraces the chemical phenomena of the circuit. Helmholtz, I believe, was the first to give the contact theory this new form, in his celebrated essay, Ueber die Erhaltung der Kraft, p. 45.

Chapter 8.

Researches on frictional electricity: induction: conduction: specific inductive capacity: theory of contiguous particles.