HUMPHRY DAVY

 

The Electrolytic Isolation of New Elements

 

Humphry Davy was born in Penzance, Cornwall, in 1778, the son of a somewhat indigent woodcarver. Davy's father died when he was a child, and his mother, Grace, supported the family by managing a millinery shop. In 1795 Davy was apprenticed to a surgeon. During his apprenticeship he threw himself into a massive project of self‑education, including languages and philosophy as well as science.

 

Evidently this all had good effects, since in 1798 he joined Beddoes's Pneumatic Institute in Bristol, as supervisor of experiments. Beddoes was the centre of a wide circle of literary and scientific acquaintances, and there Davy met both Coleridge and Southey. The former became a very close friend, and was a great influence on Davy, particularly in introducing him to the philosophy of science of Immanuel Kant. A generally Kantian standpoint exerted a great influence on Davy's ways of theorizing. While at the Pneumatic Institute he worked on a systematic study of the medicinal and therapeutic properties of gases. In 1800 he published a book on nitrous oxide (laughing gas). The work was highly successful, and made his reputation. Most of Davy's early scientific writings involved attacks on `substance' theories of physical action. Typically such theories introduced an unobservable material intermediary to explain the influence of one body on another. For instance, electrical effects were explained by postulating two electrical fluids. He was particularly severe on Lavoisier's use of the mysterious substance `caloric' to explain the phenomena of heat. True to his Kantian predilections Davy preferred theories based upon the assumptions of attractive and repulsive forces, clearly derived from the theories of R. J. Boscovich, the great Serbian theoretical physicist, and, of course, Kant.

 

In 1801 Davy was appointed Lecturer at the Royal Institution. He was an enormous success, drawing as many as a thousand people to one of his lecture‑demonstrations. He was elected a Fellow of the Royal Society in 1803, and in 1805 was awarded that society's Copley Medal for various applications of chemistry to the practical arts.

 

From about 1806 he began systematic studies in electrochemistry. He developed the uses of electrical currents as a method of analysis, as I shall describe in the text. Again this was based on a theory of attractive and repulsive forces, and the idea of a physical transport of electricity through the liquids being electrolysed. His theory shadowed forth what we should now call an `ionic' picture of electrical conduction in solutions. He was convinced that chemical affinity must have an electrical basis. Using his new methods he isolated not only potassium and sodium, but magnesium, calcium, barium, strontium, boron and silicon.

 

At that time Lavoisier's theory that oxygen was the basis of acids was still widely held. But Davy found that the oxides of his new metals were alkalis. Lavoisier's view of the role of oxygen in acidity must be astray. Part of the explanation, Davy thought, must be that the chemical properties of materials are due not only to the nature of their constituents, but to how these are arranged. By 1810 he had realized that oxygen was not a constituent of all acids. When hydrochloric acid was analysed it yielded hydrogen and another substance, erroneously thought to be an oxygen compound. Since no one, not even Davy, could break it down into constituents, of which oxygen might have been one, he concluded that it was indeed an element. And so it has proved to be. We know it as chlorine.

 

Davy had always been interested in the applications of chemistry and physics to industrial problems, and in 1812 he extended this interest by giving the first courses ever undertaken in chemistry for agriculture.

 

In 1812 he was knighted and immediately married Jane Apreece, a wealthy widow. She turned out to be a very demanding and tiresome woman, earning a great deal of animosity, not least from Michael Faraday, appointed Davy's assistant in 1813. In that year the Davys and Faraday set out on a continental tour, including a visit to Paris to receive a scientific medal from Napoleon, even though England and France were at war at the time. Lady Davy treated Faraday as a kind of lackey, something which he never forgot or forgave. Returning to England in 1815 Davy was immediately con­fronted with the task of solving the problem of explosions in mines. From this came his famous Safety Lamp.

There seems little doubt that Davy was, as we should now say, `into the drug scene'. His contacts among the poets, his own poetic ambitions, and his strongly romantic temperament all conspired to this effect. The visions described in his last but one book, Consolations in Travel, have a disconcerting famil­iarity to those who have read Casteneda and the like. Davy's health deteriorated rapidly in his middle age. After a stroke in 1827 he eked out the rest of his life in isolation and depression, moving from one European resort to another. He died in Geneva in 1829.

 

Electrolysis before Davy

 

In order for electrolysis, the decomposition of compound substances by electricity, to be a practical proposition, there had to be a readily available source of steady electric current. The birth of the idea of electrolysis and the development of the technical basis of the accumulator or battery came about together. The first step towards the discovery was Galvani's demonstration, in 1791, that electrical currents can produce muscular contractions. He noticed that a pair of frog's legs, hanging by chance in such a way that they were in contact with a junction between two dissimilar metals, twitched when the metals came into contact. In the years around 1800 Alessandro Volta carried through a systematic study of the excitation of muscular contraction and the production of electricity by the contact of dissimilar metals.

 

He was able to produce continuous quantities of electricity in a steady flow by assembling a 'pile' of coins, in dissimilar pairs, separated by cards soaked in brine, to provide the contact. The next step was to separate the metals into pairs by arranging them in a series of cups, each containing a conduct­ing solution, and connected by metal strips. This arrangement was called 'the crown of cups'. This was the first wet battery, of which our familiar lead/acid batteries are the direct descend­ants. The availability of a steady current for long periods of time allowed quite novel effects to be seen. In particular it was soon noticed that gases were produced at the electrodes, and that if the crown included a cup of water, the gases evolved were oxygen and hydrogen.

 

The experiment

 

Davy had made various attempts to decompose alkalis by passing electrical currents through aqueous solutions, but he found that 'though there was a high intensity of action, the water of the solutions alone was affected, and hydrogene and oxygene disengaged ...' If the experiment failed, that is potassium metal did not appear when potash was dissolved in water, what would happen if no water was present at all? So he tried again with molten potash. By heating 'a platinum spoon containing potash, this alkali was kept for some minutes . . . in a state of perfect fluidity'. The effects were spectacular. The spoon was connected to the positive side of the battery and the connection from the negative side was made by a platinum wire which was dipped into the molten potash. There was a bright light at the end of the negative wire and a column of flame rose above the point of contact. But when the polarity was reversed 'aeriform globules, which inflamed in the atmosphere, rose through the potash'.

It was clear to Davy that in these and similar experiments something special was being produced at the negative pole, but it could not be collected and preserved to be closely examined. 'I only attained my object', says Davy, 'by employing electri­city as the common agent for fusion and decomposition.' In the experiments with the spoon the potash had to be heated by an external flame. Though solid potash is a non‑conductor Davy found that only a little moisture was enough to make it a conductor. In that state it readily fuses and decomposes by strong electrical powers without the uncertainty of the effect of an external source of heat.

Eventually, by this last method, he succeeded. In his biography of Davy, Knight says Davy 'danced round the laboratory' when he finally succeeded in separating the globules. He put a small piece of potash, dampened only by a short exposure to the air, on a round, insulated dish of platinum which was connected to the negative pole of a battery. The positive side was connected to a platinum wire.

 

'The potash began to fuse at both its points of electrification . . . at the lower or negative surface, there was no liberation of elastic fluid [gas] but small globules, having a high metallic lustre, and being precisely similar in visible characters to quick silver, appeared, some of which burnt with explosive and bright flame, as soon as they were formed, and others remained and were merely tarnished, and finally covered by a white film which formed on their surfaces.'

At last in free form here was the substance he had been looking for, the 'basis' of potash. He soon showed that it was produced independently of the material of which the apparatus was made, so that it must be a constituent of potash. Soda exhibited an analogous result. What were these silvery globules?

 

When left in the air the metallic globules became covered with a white crust which proved to be potash re‑formed again. In pure oxygen the potash crust was formed immediately, but unless water was present to dissolve it, the crust protected the substance underneath from further attacks by oxygen. All the evidence pointed to the simplest interpretation. The experi­ment had decomposed potash and soda into distinctive 'bases' and oxygen. Davy showed that it was oxygen and only oxygen that was released at the negative pole, while oxygen and nothing else was required to turn the 'basic' back into potash or soda again. This fitted in well with Davy's electrical theory of chemical affinity. As he summed it up, 'The combustible bases of the fixed alkalis seem to be repelled as [are] other combustible substances, by positively charged surfaces, and attracted by negatively electrified surfaces, and the oxygen follows the contrary order.' But when they come together in the synthesis of the potash, 'the natural energies or attractions come in equilibrium with each other'.

The migrations of ions

 

Eventually, by patiently collecting the globules (facilitated by his discovery that they did not react with naphtha), Davy made up samples of the new substances. He found that they were good conductors of heat and electricity. But though they resembled metals in all their major properties they were exceptionally light. Their specific gravity was actually less than that of water. They were chemically very active, particularly in reaction with water. He describes several experiments in which these substances seemed almost to be actively seeking out traces of water with which to combine. In a similar way they seemed to hunt oxygen. They would readily reduce, that is extract the oxygen from, other metallic oxides. After consult­ing the opinions of a number of philosophically minded persons, Davy decided that these 'bases' were indeed metals. He chose to call them 'Potasium' and 'Sodium'. He quickly altered the spelling of the former .to our modern 'potassium'. The derivation of these names, he remarks, is 'perhaps more significant than elegant'. But they have 'the great advantage that whether changes occur in the theory of the composition of metals these terms will remain good, for all they mean is the metal derived from potash and from soda respectively'. Dave thought that one should be rather cautious in using terms that were redolent of theory, particularly at a time when discoveries in the electrochemical field were coming so thick and fast.

Caution, too, showed in his observation as to whether these were elementary substances. Probably they were, but all one could say was 'we have no good reason for assuming the compound nature of this class of bodies'.

 

One further conclusion of consequence could be drawn from the result of this experiment and the testing of the new metal and their chemical properties, and from the study of am­monium hydroxide. There was oxygen in all the alkalis, and in the fixed alkalis, potash and soda, there seemed to be nothing but the metal and oxygen. But Lavoisier had thought that oxygen was the principle of acidity, and indeed that is what the word `oxygen' had meant. However, says Davy, `Oxygen then may be considered as existing in, and as forming, an element in all true alkalis; and the principle of acidity of the French nomenclature, might now likewise be called the principle of alkalescence.'

 

Electrolysis after Davy

 

Further developments of electrolytic methods of decomposi­tion were mostly restricted to industrial applications. The separation of new elements became more and more a matter of chemical analysis. Great analytical sagas, like that of the Curies' separation of radium, were based on finding chemical reactions by which the differential solubilities of corresponding compounds of the elementary substances involved could be exploited to separate them.

If Lavoisier's experiments with oxygen were a case of `cap­fitting', given the head find the right cap for it, Davy's could be thought of as `bill‑filling', given a prior prescription of what to expect (a light, active metal) how can we find something to fill it? The electrochemical theory had convinced Davy that light metals must be the bases of the oxides. And that theory dictated the means by which they might be released.

 

Further reading

 

Davy, H., `The Bakerian Lecture', Philosophical Transactions of the Royal Society, Part I, 1808, pp. 1‑44.

Davy, J., Memoirs of the Life of Sir Humphry Davy, Bart., London, 1836.

Hartley, H., Humphry Davy, London, 1967.

Knight, D. M., 'Davy' in Gillispie, C. C. (ed.), Dictionary of Scientific Biography, vol. 3, New York, 1971.