The distinction at first drawn between "earth" and "alkali" was too absolute; the intermediate group of "alkaline earths" served to bridge over the gap between the extreme groups.

"In Nature," says Wordsworth, "everything is distinct, but nothing defined into absolute independent singleness."

At this stage of advance, then, an earth is regarded as differing from an alkali in being insoluble, or nearly insoluble in water; in not being soapy to the touch, and not turning vegetable reds to blue: but as resembling an alkali, in that it combines with and neutralizes an acid; and the product of this neutralization, whether accomplished by an alkali or by an earth, is called a salt. To the earth or alkali, as being the foundation on which the salt is built, by the addition of acid, the name of base was given by Rouelle in 1744.

But running through every conception which was formed of these substances—acid, alkali, earth, salt—we find a tendency, sometimes forcibly marked, sometimes feebly indicated, but always present, to consider salt as a term of much wider acceptation than any of the others. An acid and an alkali, or an acid and an earth, combine to form a salt; but the salt could not have been thus produced unless the acid, the alkali and the earth had contained in themselves some properties which, when combined, form the properties of the salt.

The acid, the alkali, the earth, each is, in a sense, a salt. The perfect salt is produced by the coalescence of the saltness of the acid with the saltness of the alkali. This conception finds full utterance in the names, once in common use, of sal acidum for acid, sal alkali for alkali, and sal salsum or sal neutrum for salt. All are salts; at one extreme comes that salt which is marked by properties called acid properties, at the other extreme comes the salt distinguished by alkaline properties, and between these, and formed by the union of these, comes the middle or neutral salt.

It is thus that the nomenclature of chemistry marks the advances made in the science. "What's in a name?" To the historical student of science, almost everything.

We shall find how different is the meaning attached in modern chemistry to these terms, acid salt, alkaline salt, neutral salt, from that which our predecessors gave to their sal acidum, sal alkali, and sal neutrum.

We must note the appearance of the term vitriol, applied to the solid salt-like bodies obtained from acids and characterized by a glassy lustre. By the middle of last century the vitriols were recognized as all derived from, or compounded of, sulphuric acid (oil of vitriol) and metals; this led to a subdivision of the large class of neutral salts into (1) metallic salts produced by the action of sulphuric acid on metals, and (2) neutral salts produced by the action of earths or alkalis on acids generally.

To Rouelle, a predecessor of Lavoisier, who died four years before the discovery of oxygen, we owe many accurate and suggestive remarks and experiments bearing on the term "salt." I have already mentioned that it was he who applied the word "base" to the alkali or earth, or it might be metal, from which, by the action of acid, a salt is built up. He also ceased to speak of an acid as sal acidum, or of an alkali as sal alkali, and applied the term "salt" exclusively to those substances which are produced by the action of acids on bases. When the product of such an action was neutral—that is, had no sour taste, no soapy feeling to the touch, no action on vegetable colours, and no action on acids or bases—he called that product a neutral salt; when the product still exhibited some of the properties of acid, e.g. sourness of taste, he called it an acid salt; and when the product continued to exhibit some of the properties of alkali, e.g. turned vegetable reds to blue, he called it an alkaline salt.

Rouelle also proved experimentally that an acid salt contains more acid—relatively to the same amount of base—than a neutral salt, and that an alkaline salt contains more base—relatively to the same amount of acid—than a neutral salt; and he proved that this excess of acid, or of base, is chemically united to the rest of the salt—is, in other words, an essential part of the salt, from which it cannot be removed without changing the properties of the whole.

But we have not as yet got to know why certain qualities connoted by the term "acid" can be affirmed to belong to a group of bodies, why certain other, "alkaline," properties belong to another group, nor why a third group can be distinguished from both of these by the possession of properties which we sum up in the term "earthy." Surely there must be some peculiarity in the composition of these substances, common to all, by virtue of which all are acid. The atom of an acid is surely composed of certain elements which are never found in the atom of an alkali or an earth; or perhaps the difference lies in the number, rather than in the nature of the elements in the acid atoms, or even in the arrangement of the elementary atoms in the compound atom of acid, of alkali, and of earth.

I think that our knowledge of salt is now more complete than our knowledge of either acid, alkali, or earth. We know that a salt is formed by the union of an acid and an alkali or earth; if, then, we get to know the composition of acids and bases (i.e. alkalis and earths), we shall be well on the way towards knowing the composition of salts.

And now we must resume our story where we left it at p. 176. Lavoisier had recognized oxygen as the acidifier; Black had proved that a caustic alkali does not contain carbonic acid.

Up to this time metallic calces, and for the most part alkalis and earths also, had been regarded as elementary substances. Lavoisier however proved calces to be compounds of metals and oxygen; but as some of those calces had all the properties which characterized earths, it seemed probable that all earths are metallic oxides, and if all earths, most likely all alkalis also. Many attempts were made to decompose earths and alkalis, and to obtain the metal, the oxide of which the earth or the alkali was supposed to be. One chemist thought he had obtained a metal by heating the earth baryta with charcoal, but from the properties of his metal we know that he had not worked with a pure specimen of baryta, and that his supposed metallic base of baryta was simply a little iron or other metal, previously present in the baryta, or charcoal, or crucible which he employed.

But if Lavoisier's view were correct—if all bases contained oxygen—it followed that all salts are oxygen compounds. Acids all contain oxygen, said Lavoisier; this was soon regarded as one of the fundamental facts of chemistry. Earths and alkalis are probably oxides of metals; this before long became an article of faith with all orthodox chemists. Salts are produced by the union of acids and bases, therefore all salts contain oxygen: the conclusion was readily adopted by almost every one.

When the controversy between Lavoisier and the phlogistic chemists was at its height, the followers of Stahl had taunted Lavoisier with being unable to explain the production of hydrogen (or phlogiston as they thought) during the solution of metals in acids; but when Lavoisier learned the composition of water, he had an answer sufficient to quell these taunts. The metal, said Lavoisier, decomposes the water which is always present along with the acid, hydrogen is thus evolved, and the metallic calx or oxide so produced dissolves in the acid and forms a salt. If this explanation were correct—and there was an immense mass of evidence in its favour and apparently none against it—then all the salts produced by the action of acids on metals necessarily contained oxygen.

The Lavoisierian view of a salt, as a compound of a metallic oxide—or base—with a non-metallic oxide—or acid—seemed the only explanation which could be accepted by any reasonable chemist: in the early years of this century it reigned supreme.

But even during the lifetime of its founder this theory was opposed and opposed by the logic of facts. In 1787 Berthollet published an account of experiments on prussic acid,—the existence and preparation (from Prussian blue) of which acid had been demonstrated three or four years before by the Swedish chemist Scheele—which led him to conclude this compound to be a true acid, but free from oxygen. In 1796 the same chemist studied the composition and properties of sulphuretted hydrogen, and pronounced this body to be an acid containing no oxygen.

But the experiments and reasoning of Berthollet were hidden by the masses of facts and the cogency of argument of the Lavoisierian chemists.

The prevalent views regarding acids and bases were greatly strengthened by the earlier researches of Sir Humphry Davy, in which he employed the voltaic battery as an instrument in chemical investigation. Let us now consider some of the electro-chemical work of this brilliant chemist.

In the spring of the year 1800 the electrical battery, which had recently been discovered by Volta, was applied by Nicholson and Carlisle to effect the decomposition of water. The experiments of these naturalists were repeated and confirmed by Davy, then resident at Bristol, who followed up this application of electricity to effect chemical changes by a series of experiments extending from 1800 to 1806, and culminating in the Bakerian Lecture delivered before the Royal Society in the latter year.

The history of Davy's life during these years, years rich in results of the utmost importance to chemical science, will be traced in the sequel; meanwhile we are concerned only with the results of his chemical work.

The first Bakerian Lecture of Humphry Davy, "On some Chemical Agencies of Electricity," deserves the careful study of all who are interested in the methods of natural science; it is a brilliant example of the disentanglement of a complex natural problem.

Volta and others had subjected water to the action of a current of electricity, and had noticed the appearance of acid and alkali at the oppositely electrified metallic surfaces. According to some experimenters, the acid was nitrous, according to others, muriatic acid. One chemist asserted the production of a new and peculiar body which he called the electric acid. The alkali was generally said to be ammonia.

When Davy passed an electric current through distilled water contained in glass vessels, connected by pieces of moist bladder, cotton fibre, or other vegetable matters, he found that nitric and hydrochloric acids were formed in the water surrounding the positively electrified plate or pole, and soda around the negatively electrified pole, of the battery.

When the same piece of cotton fibre was repeatedly used for making connection between the glass vessels, and was washed each time in dilute nitric acid, Davy found that the production of muriatic acid gradually ceased; hence he traced the formation of this acid to the presence of the animal or vegetable substance used in the experiments.

Finding that the glass vessels were somewhat corroded, and that the greater the amount of corrosion the greater was the amount of soda making its appearance around the negative pole, he concluded that the soda was probably a product of the decomposition of the glass by the electric current; he therefore modified the experiment. He passed an electric current through distilled water contained in small cups of agate, previously cleaned by boiling in distilled water for several hours, and connected by threads of the mineral asbestos, chosen as being quite free from vegetable matter; alkali and acid were still produced. The experiment was repeated several times with the same apparatus; acid and alkali were still produced, but the alkali decreased each time. The only conclusion to be drawn was that the alkali came from the water employed. Two small cups of gold were now used to contain the water; a very small amount of alkali appeared at the negative pole, and a little nitric acid at the positive pole. The quantity of acid slowly increased as the experiment continued, whereas the quantity of alkali remained the same as after a few minutes' action of the electric current. The production of alkali is probably due, said Davy, to the presence in the water of some substance which is not removed by distillation in a glass retort. By boiling down in a silver dish a quantity of the water he had used, a very small amount of solid matter was obtained, which after being heated was distinctly alkaline. Moreover when a little of this solid matter was added to the water contained in the two golden cups, there was a sudden and marked increase in the amount of alkali formed around the negative pole. Another quantity of the water which he had used was again distilled in a silver retort, and a little of the distillate was subjected to electrolysis as before. No alkali appeared. A little piece of glass was placed in the water; alkali quickly began to form. Davy thus conclusively proved that the alkali produced during the electrolysis (i.e. decomposition by the electric current) of water is not derived from the water itself, but from mineral impurities contained in the water, or in the vessel in which the water is placed during the experiment. But the production of nitric acid around the positive pole was yet to be accounted for.

Before further experiments could be made it was necessary that Davy should form an hypothesis—that he should mentally connect the appearance of the nitric acid with some other phenomenon sufficient to produce this appearance; he could then devise experiments which would determine whether the connection supposed to exist between the two phenomena really did exist or not.

Now, of the constituents of nitric acid—nitrogen, hydrogen and oxygen—all except the first named are present in pure water; nitrogen is present in large quantity in the ordinary atmosphere. It was only necessary to assume that some of the hydrogen and oxygen produced during the electrolysis of water seized on and combined with some of the nitrogen in the air which surrounded that water, and the continual production of nitric acid during the whole process of electrolysis was explained.

But how was this assumption to be proved or disproved? Davy adopted a method frequently made use of in scientific investigations:—remove the assumed cause of a phenomenon; if the phenomenon ceases to be produced, the assumed cause is probably the real cause. Davy surrounded the little gold cups containing the water to be electrolysed with a glass jar which he connected with an air-pump; he exhausted most of the air from the jar and then passed the electric current through the water. Very little nitric acid appeared. He now again took out most of the air from the glass jar, admitted some hydrogen to supply its place, and again pumped this out. This process he repeated two or three times and then passed the electric current. No acid appeared in the water. He admitted air into the glass vessel; nitric acid began to be produced. Thus he proved that whenever air was present in contact with the water being electrolysed, nitric acid made its appearance, and when the air was wholly removed the acid ceased to be produced. As he had previously shown that the production of this acid was not to be traced to impurities in the water, to the nature of the vessel used to contain the water, or to the nature of the material of which the poles of the battery were composed, the conclusion was forced upon him that the production of nitric acid in the water, and the presence of ordinary air around the water invariably existed together; that if one of these conditions was present, the other was also present—in other words, that one was the cause of the other.

The result of this exhaustive and brilliant piece of work is summed up by Davy in these words: "It seems evident then that water, chemically pure, is decomposed by electricity into gaseous matter alone, into oxygen and hydrogen."

From the effects of the electric current on glass, Davy argued that other earthy compounds would probably undergo change under similar conditions. He therefore had little cups of gypsum made, in which he placed pure water, and passed an electric current through the liquid. Lime was formed around the negative, and sulphuric acid around the positive pole. Using similar apparatus, he proved that the electric current decomposes very many minerals into an earthy or alkaline base and an acid.

Picturing to himself the little particles of a salt as being split by the electric current each into two smaller particles, one possessed of acid and the other of alkaline properties, Davy thought it might be possible to intercept the progress of these smaller particles, which he saw ever travelling towards the positive and negative poles of the battery. He accordingly connected these small glass vessels by threads of washed asbestos; in one of the outer vessels he placed pure water, in the other an aqueous solution of sulphate of potash, and in the central vessel he placed ammonia. The negative pole of the battery being immersed in the sulphate of potash, and the positive pole in the water, it was necessary for the particles of sulphuric acid—produced by the decomposition of the sulphate of potash—to travel through the ammonia in the central vessel before they could find their way to the positive pole. Now, ammonia and sulphuric acid cannot exist in contact—they instantly combine to form sulphate of ammonia; the sulphuric acid particles ought therefore to be arrested by the ammonia. But the sulphuric acid made its appearance at the positive pole just as if the central vessel had contained water. It seemed that the mutual attraction ordinarily exerted between sulphuric acid and ammonia was overcome by the action of the electric current. Ammonia would generally present an insuperable barrier to the progress of sulphuric acid, but the electrical energy appeared to force the acid particles over this barrier; they passed towards their goal as if nothing stood in their way.

Experiments are now multiplied by Davy, and the general conclusion drawn is that "Hydrogen, the alkaline substances, the metals and certain metallic oxides are attracted by negatively electrified metallic surfaces, and repelled by positively electrified metallic surfaces; and contrariwise, that oxygen and acid substances are attracted by positively electrified metallic surfaces, and repelled by negatively electrified metallic surfaces; and these attractive and repulsive forces are sufficiently energetic to destroy or suspend the usual operation of chemical affinity."[10]

To account for this apparent suspension of the ordinary chemical laws, Davy supposes that chemical compounds are continually decomposed and re-formed throughout the liquid which is subjected to the electrical action. Thus, in the experiment with water, ammonia and sulphate of potash, he supposes that the sulphuric acid and ammonia do combine in the central vessel to form sulphate of ammonia, but that this compound is again decomposed, by the electrical energy, into sulphuric acid—which passes on towards the positive pole—and ammonia—which remains in the central vessel—ready to combine with more sulphuric acid as that comes travelling onwards from its source in the vessel containing sulphate of potash to its goal in the vessel containing water.

The eye of the philosopher had pierced beneath the apparent stability of the chemical systems which he studied. To his vision there appeared in those few drops of water and ammonia and sulphate of potash a never-ceasing conflict of contending forces; there appeared a continual shattering and rebuilding of the particles of which the masses were composed. The whole was at rest, the parts were in motion; the whole was constant in chemical composition, the composition of each particle was changed a thousand times in the minutest portion of every second. To the mind of Davy, the electrolysis of every chemical compound was a new application of the great law established by Newton—"To every action there is an equal and opposite reaction."

Each step made in chemical science since Davy's time has but served to emphasize the universality of this principle of action and reaction, a principle which has been too much overlooked in the chemical text-books, but the importance of which recent researches are beginning to impress on the minds of chemists.

It is the privilege of the philosophic student of Nature to penetrate the veil with which she conceals her secrets from the vulgar gaze. To him are shown sights which "eye hath not seen," and by him are perceived sounds which "ear hath not heard." Each drop of water is seen by him not only to be built up of myriads of small parts, but each particle is seen to be in motion; many particles are being decomposed into still smaller particles of matter, different in properties from the original particles, but as the original particles are at the same time being reproduced, the continued existence of the drop of water with the properties of water is to him the result of the mutual action and reaction of contending forces. He knows that rest and permanence are gained, not by the cessation of action, but by the continuance of conflict; he knows that in the realm of natural phenomena, stable equilibrium is the resultant of the action of opposite forces, and that complete decomposition occurs only when one force becomes too powerful or another becomes too weak.

Pursuing the train of thought initiated by the experiments which I have described, Davy entered upon a series of researches which led him to consider every chemical substance as possessing definite electrical relations towards every other substance. "As chemical attraction between two bodies seems to be destroyed by giving one of them an electrical state different from that which it naturally possessed—that is, by bringing it into a state similar to the other—so it may be increased by exalting its natural energy." Thus zinc, a metal easily oxidized, does not combine with oxygen when negatively electrified, whereas silver, a metal oxidized with difficulty, readily combines with oxygen when positively electrified.

Substances in opposite electrical states appear to combine chemically, and the greater the electrical difference the greater the readiness with which chemical combination is effected. Electrical energy and chemical attraction or affinity are evidently closely connected; perhaps, said Davy, they are both results of the same cause.

Thus Davy arrived at the conception of a system of bodies as maintained in equilibrium by the mutual actions and reactions of both chemical and electrical forces; by increasing either of these a change is necessarily produced in the other. Under certain electrical conditions the bodies will exert no chemical action on one another, but such action may be started by changing these electrical conditions, or, on the other hand, by changes in the chemical relations of the bodies a change in the electrical relations may be induced. Thus Davy found that if plates of copper and sulphur are heated, the copper exhibits a positive and the sulphur a negative electrical condition; that these electrical states become more marked as temperature rises, until the melting point of sulphur is reached, when the copper and sulphur combine together chemically and produce sulphide of copper.

When water is electrolysed, Davy looked on the oppositely electrified metallic plates in the battery as striving to attain a state of equilibrium; the negatively electrified zinc strives to gain positive electricity from the copper, which strives to gain negative electricity from the zinc. The water he regarded as the carrier of these electricities, the one in this direction, the other in that. In thus acting as a carrier, the water is itself chemically decomposed, with production of hydrogen and oxygen; but this chemical rearrangement of some of the substances which composed the original system (of battery and water) involves a fresh disturbance of electrical energy, and so the process proceeds until the whole of the water is decomposed or the whole of the copper or zinc plate is dissolved in the battery. If the water were not chemically decomposed, Davy thought that the zinc and copper in the battery would quickly attain the state of electrical equilibrium towards which they continually strive, and that the current would therefore quickly cease.

Davy thought that "however strong the natural electrical energies of the elements of bodies may be, yet there is every probability of a limit to their strength; whereas the powers of our artificial instruments seem capable of indefinite increase." By making use of a very powerful battery, he hoped to be able to decompose substances generally regarded as simple bodies.

Taking a wide survey of natural phenomena, he sees these two forces, which we call chemical and electrical, everywhere at work, and by their mutual actions upholding the material universe in equilibrium. In the outbreaks of volcanoes he sees the disturbance of this equilibrium by the undue preponderance of electrical force; and in the formation of complex minerals beneath the surface of the earth, he traces the action of those chemical attractions which are ever ready to bring about the combination of elements, if they are not held in check by the opposing influence of electrical energy.

We shall see how the great and philosophical conception of Davy was used by Berzelius, and how, while undoubtedly gaining in precision, it lost much in breadth in being made the basis of a rigid system of chemical classification.

Davy's hope that the new instrument of research placed in the hands of chemists by Volta would be used in the decomposition of supposed simple substances was soon to be realized. A year after the lecture "On some Chemical Agencies of Electricity," Davy was again the reader of the Bakerian Lecture; this year (1807) it was entitled, "On some New Phenomena of Chemical Change produced by Electricity, particularly the Decomposition of the Fixed Alkalis; and the Exhibition of the New Substances which constitute their Bases; and on the General Nature of Alkaline Bodies."

In his first experiments on the effect of the electrical current on potash and soda, Davy used strong aqueous solutions of these alkalis, with the result that hydrogen and oxygen only were evolved. He then passed the current through melted potash kept liquid during the operation by the use of a spirit-lamp, the flame of which was fed with oxygen. Much light was evolved, and a great flame appeared at the negative pole; on changing the direction of the current, "aeriform globules, which inflamed in the air, rose through the potash."

On the 6th of October 1807, a piece of potash was placed on a disc of platinum, which was made the negative pole of a very powerful battery; a platinum wire brought into contact with the upper surface of the potash served as the positive pole. When the current was passed, the potash became hot and soon melted; gas was evolved at the upper surface, and at the lower (negative) side "there was no liberation of elastic fluid, but small globules, having a high metallic lustre, and being precisely similar in visible characters to quicksilver appeared, some of which burst with explosion 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."

When Davy saw these metallic globules burst through the crust of fusing potash, we are told by one of his biographers, "he could not contain his joy, he actually bounded about the room in ecstatic delight; and some little time was required for him to compose himself sufficiently to continue the experiment."

This was the culminating point of the researches in which he had been continuously engaged for about six years. His interest and excitement were intense; the Bakerian Lecture was written "on the spur of the occasion, before the excitement of the mind had subsided," yet, says his biographer—and we may well agree with him—"yet it bears proof only of the maturest judgment; the greater part of it is as remarkable for experimental accuracy as for logical precision." But "to every action there is an equal and opposite reaction:" immediately after the delivery of the lecture, Davy was prostrated by a severe attack of illness, which confined him to bed for nine weeks, and was very nearly proving fatal.

That the phenomenon just described was really the decomposition of potash, and the production of the metal of which this substance is an oxygenized compound, was proved by obtaining similar results whether plates of silver, copper, or gold, or vessels of plumbago, or even charcoal, were used to contain the potash, or whether the experiment was conducted in the air, or in a glass vessel from which air had been exhausted, or in glass tubes wherein the potash was confined by mercury. The decomposition of potash was followed within a few days by that of soda, from which substance metallic globules were obtained which took fire when exposed to the air.

But the analysis of potash and soda was not sufficient for Davy; he determined to accomplish the synthesis of these substances. For this purpose he collected small quantities of the newly discovered metals, by conducting the electrolysis of potash and soda under experimental conditions such that the metals, as soon as produced, were plunged under the surface of naphtha, a liquid which does not contain oxygen, and which protected them from the action of the surrounding air.

A weighed quantity of each metal was then heated in a stream of pure dry oxygen, the products were collected and weighed, and it was found that solutions of these products in water possessed all the properties of aqueous solutions of potash and soda.

The new metals were now obtained in larger quantity by Davy, and their properties carefully determined by him; they were named potassium and sodium respectively. They were shown to possess all those properties which were generally accepted as characteristic of metal, except that of being heavy. The new metals were extremely light, lighter than water. For some time it was difficult to convince all chemists that a metal could be a very light substance. We are assured that a friend of Davy, who was shown potassium for the first time, and was asked what kind of substance he supposed it to be, replied, "It is metallic, to be sure;" "and then, balancing it on his finger, he added in a tone of confidence, 'Bless me, how heavy it is!'"

Davy argued that since the alkalis, potash and soda, were found to be oxygen compounds of metals, the earths would probably also be found to be metallic oxides. In the year 1808 he succeeded in decomposing the three earths, lime, baryta and strontia, and in obtaining the metals calcium, barium and strontium, but not in a perfectly pure condition, or in any quantity. He also got evidence of the decomposition of the earths silica, alumina, zirconia and beryllia, by the action of powerful electric currents, but he did not succeed in obtaining the supposed metallic bases of these substances.

So far Davy's discoveries had all tended to confirm the generally accepted view which regarded alkalis and earths as metallic oxides. But we found that the outcome of these views was to regard all salts—and among these, of course, common salt—as oxygen compounds.[11] Acids were oxygen compounds, bases were oxygen compounds, and as salts were produced by the union of acids with bases, they, too, must necessarily be oxygen compounds.

Berthollet had thrown doubt on the universality of Lavoisier's name "oxygen," the acidifier, but he had not conclusively proved the existence of any acid which did not contain oxygen.

The researches of Davy naturally led him to consider the prevalent views regarding acids, bases and salts.

Muriatic (or as we now call it hydrochloric) acid had long been a stumbling-block to the thorough-going Lavoisierian chemists. Oxygen could not be detected in it, yet it ought to contain oxygen, because oxygen is the acidifier. Of course, if muriatic acid contains oxygen, the salts—muriates—produced by the action of this acid on alkalis and earths must also contain oxygen. Many years before this time the action of muriatic acid on manganese ore had been studied by the Swedish chemist Scheele, who had thus obtained a yellow-coloured gas with a very strong smell. Berthollet had shown that when a solution of this gas in water is exposed to sunlight, oxygen is evolved and muriatic acid is produced. The yellow gas was therefore supposed to be, and was called, "oxidized muriatic acid," and muriatic acid was itself regarded as composed of oxygen and an unknown substance or radicle.

In 1809 Gay-Lussac and Thenard found that one volume of hydrogen united with one volume of the so-called oxidized muriatic acid to form muriatic acid; the presence of hydrogen in this acid was therefore proved.

When Davy began (1810-11) to turn his attention specially to the study of salts, he adopted the generally accepted view that muriatic acid is a compound of oxygen and an unknown radicle, and that by the addition of oxygen to this compound oxidized muriatic acid is produced. But unless Davy could prove the presence of oxygen in muriatic acid he could not long hold the opinion that oxygen was really a constituent of this substance. He tried to obtain direct evidence of the presence of oxygen, but failed. He then set about comparing the action of muriatic acid on metals and metallic oxides with the action of the so-called oxidized muriatic acid on the same substances. He showed that salt-like compounds were produced by the action of oxidized muriatic acid either on metals or on the oxides of these metals, oxygen being evolved in the latter cases; and that the same compounds and water were produced by the action of muriatic acid on the same metallic oxides.

These results were most easily and readily explained by assuming the so-called oxidized muriatic acid to be an elementary substance, and muriatic acid to be a compound of this element with hydrogen. To the new element thus discovered—for he who establishes the elementary nature of a substance may almost be regarded as its discoverer—Davy gave the name of chlorine, suggested by the yellow colour of the gas (from Greek, = yellow). He at once began to study the analogies of chlorine, to find by experiment which elements it resembled, and so to classify it. Many metals, he found, combined readily with chlorine, with evolution of heat and light. It acted, like oxygen, as a supporter of combustion; it was, like oxygen, attracted towards the negative pole of the voltaic battery; its compound with hydrogen was an acid; hence said Davy chlorine, like oxygen, is a supporter of combustion and also an acidifier.

But it was very hard to get chemists to adopt these views. As Bacon says, "If false facts in Nature be once on foot, what through neglect of examination, the countenance of antiquity, and the use made of them in discourse, they are scarce ever retracted."

Chemists had long been accustomed to systems which pretended to explain all chemical facts. The phlogistic theory, which had tyrannized over chemistry, had been succeeded by the Lavoisierian chemistry, which recognized one acidifier, and this also the one supporter of combustion. To ascribe these properties to any element other than oxygen appeared almost profane.

But when Davy spoke of chlorine as an acidifier, he did not use this word in the same sense as that in which it was employed by the upholders of the oxygen theory of acids; he simply meant to express the fact that a compound containing chlorine as one of its constituents, but not containing oxygen, was a true acid. When Gay-Lussac attempted to prove that hydrogen is an alkalizing principle, Davy said, "This is an attempt to introduce into chemistry a doctrine of occult qualities, and to refer to some mysterious and inexplicable energy what must depend upon a peculiar corpuscular arrangement." And with regard to Gay-Lussac's strained use of analogies between hydrogen compounds and alkalis, he says, "The substitution of analogy for fact is the bane of chemical philosophy; the legitimate use of analogy is to connect facts together, and to guide to new experiments."

But Davy's facts were so well established, and his experiments so convincing, that before two or three years had passed, most chemists were persuaded that chlorine was an element—i.e. a substance which had never been decomposed—and that muriatic acid was a compound of this element with hydrogen.

Berzelius was among the last to adopt the new view. Wöhler tells us that in the winter of 1823, when he was working in the laboratory of Berzelius, Anna, while washing some basins, remarked that they smelt strongly of oxidized muriatic acid: "Now," said Berzelius, "listen to me, Anna. Thou must no longer say 'oxidized muriatic acid,' but 'chlorine;' that is better."

This work on chlorine was followed up, in 1813, by the proof that the class of acidifiers and supporters of combustion contains a third elementary substance, viz. iodine. As Davy's views regarding acids and salts became developed, he seems to have more and more opposed the assumption that any one element is especially to be regarded as the acidifying element; but at the same time he seems to admit that most, if not all, acids contain hydrogen. Such oxides as sulphur trioxide, nitrogen pentoxide, etc., do not possess acid properties except in combination with water. But he of course did not say that all hydrogen compounds are acids; he rather regarded the possession by a substance of acid properties as dependent, to a great extent, on the nature of the elements other than hydrogen which it contained, or perhaps on the arrangement of all the elements in the particles of the acid. He regarded the hydrogen in an acid as capable of replacement by a metal, and to the metallic derivative—as it might be called—of the acid, thus produced, he gave the name of "salt." An acid might therefore be a compound of hydrogen with one other element—such were hydrochloric, hydriodic, hydrofluoric acids—or it might be a compound of hydrogen with two or more elements, of which one might or might not be oxygen—such were hydrocyanic acid and chloric or nitric acid. If the hydrogen in any of these acids were replaced by a metal a salt would be produced. A salt might therefore contain no oxygen, e.g. chloride or iodide of potassium; but in most cases salts did contain oxygen, e.g. chlorate or nitrate of potassium.

Acids were thus divided into oxyacids (or acids which contain oxygen) and acids containing no oxygen; the former class including most of the known acids. The old view of salts as being compounds of acids (i.e. oxides of the non-metallic elements) and bases (i.e. oxides of metals) was overthrown, and salts came to be regarded as metallic derivatives of acids.

From this time, these terms—acids, salts, bases—become of less importance than they formerly were in the history of chemical advance.

In trying to explain Davy's electro-chemical theory I have applied the word affinity to the mutual action and reaction between two substances which combine together to form a chemical compound. It is now necessary that we should look a little more closely into the history of this word affinity.

Oil and water do not mix together, but oil and potash solution do; the former may be said not to have, and the latter to have, an affinity one for the other. When sulphur is heated, the yellow odourless solid, seizing upon oxygen in the air, combines with it to produce a colourless strongly smelling gas. Sulphur and oxygen are said to have strong affinity for each other.

If equal weights of lime and magnesia be thrown into diluted nitric acid, after a time it is found that some of the lime, but very little of the magnesia, is dissolved. If an aqueous solution of lime be added to a solution of magnesia in nitric acid, the magnesia is precipitated in the form of an insoluble powder, while the lime remains dissolved in the acid. It is said that lime has a stronger affinity for nitric acid than magnesia has. Such reactions as these used to be cited as examples of single elective affinity—single, because one substance combined with one other, and elective, because a substance seemed to choose between two others presented to it, and to combine with one to the exclusion of the other.

But if a neutral solution of magnesia in sulphuric acid is added to a neutral solution of lime in nitric acid, sulphate of lime and nitrate of magnesia are produced. The lime, it was said, leaves the nitric and goes to the sulphuric acid, which, having been deserted by the magnesia, is ready to receive it; at the same time the nitric acid from which the lime has departed combines with the magnesia formerly held by the sulphuric acid. Such a reaction was said to be an instance of double affinities. The chemical changes were caused, it was said, by the simultaneous affinity of lime for sulphuric acid, which was greater than its affinity for nitric acid, and the affinity of magnesia for nitric acid, which was greater than its affinity for sulphuric acid.

If a number of salts were mixed, each base—supposing the foregoing statements to be correct—would form a compound with that acid for which it had the greatest affinity. It should then be possible to draw up tables of affinity. Such tables were indeed prepared. Here is an example:—

This table tells us that the affinity of baryta for sulphuric acid is greater than that of strontia for the same acid, that of strontia greater than that of potash, and so on. It also tells that potash will decompose a compound of sulphuric acid and soda, just as soda will decompose a compound of the same acid with lime, or strontia will decompose a compound with potash, etc.

But Berthollet showed in the early years of this century that a large quantity of a body having a weak affinity for another will suffice to decompose a small quantity of a compound of this other with a third body for which it has a strong affinity. He showed, that is, that the formation or non-formation of a compound is dependent not only on the so-called affinities between the constituents, but also on the relative quantities of these constituents. Berthollet and other chemists also showed that affinity is much conditioned by temperature; that is, that two substances which show no tendency towards chemical union at a low temperature may combine when the temperature is raised. He, and they, also proved that the formation or non-formation of a compound is much influenced by its physical properties. Thus, if two substances are mixed in solution, and if by their mutual action a substance can be produced which is insoluble in the liquids present, that substance is generally produced whether the affinity between the original pair of substances be strong or weak.

The outcome of Berthollet's work was that tables of affinity became almost valueless. To say that the affinity of this body for that was greater than its affinity for a third body was going beyond the facts, because the formation of this or that compound depended on many conditions much more complex than those connoted by the term "affinity." Yet the conception of affinity remained, although it could not be applied in so rigorous a way as had been done by the earlier chemists. If an element, A, readily combines with another element, B, under certain physical conditions, but does not, under the same conditions, combine with a third element, C, it may still be said that A and B have, and A and C have not, an affinity for each other.

This general conception of affinity was applied by Berzelius to the atoms of elements. Affinity, said Berzelius, acts between unlike atoms, and causes them to unite to form a compound atom, unlike either of the original atoms; cohesion, on the other hand, acts between like atoms, causing them to hold together without producing any change in their properties. Affinity varies in different elements. Thus the affinity of gold for oxygen is very small; hence it is that gold is found in the earth in the metallic state, while iron, having a great affinity for oxygen, soon rusts when exposed to air, or when buried in the earth. Potassium and sodium have great affinities for oxygen, chlorine, etc.; yet the atoms of potassium and sodium do not themselves combine. The more any elements are alike chemically the smaller is their affinity for each other; the more any elements are chemically unlike the greater is their mutual affinity; but this affinity is modified by circumstances. Thus, said Berzelius, if equal numbers of atoms of A and B, having equal or nearly equal affinity for C, mutually react, compound atoms, AC and BC, will be produced, but atoms of A and B will remain. The amounts of AC and BC produced will be influenced by the greater or less affinity of A and B for C; but if there be a greater number of A than of B atoms, a greater amount of AC than of BC will be produced. In these cases all the reacting substances and the products of the actions are supposed to be liquids; but BC, if a solid substance, will be produced even if the affinity of A for C is greater than that of B for C.

In some elements, Berzelius taught, affinity slumbers, and can be awakened only by raising the temperature. Thus carbon in the form of coal has no affinity for oxygen at ordinary temperatures; it has remained for ages in the earth without undergoing oxidation; but when coal is heated the affinities of carbon are awakened, combination with oxygen occurs, and heat is produced.

But why is it that certain elementary atoms exhibit affinity for certain others? It depends, said Berzelius, on the electrical states of these atoms. According to the Berzelian theory, every elementary atom has attached to it a certain quantity of electricity, part of which is positive and part negative. This electricity is accumulated at two points on each atom, called respectively the positive pole and the negative pole; but in each atom one of these electricities so much preponderates over the other as to give the whole atom the character of either a positively or a negatively electrified body. When two atoms combine chemically the positive electricity in one neutralizes the negative electricity in the other. As we know that similar electricities repel, and opposite electricities attract each other, it follows that a markedly positive atom will exhibit strong affinity for a markedly negative atom, less strong affinity for a feebly negative, and little or no affinity for a positively electrified atom; but two similarly electrified atoms may exhibit affinity, because in every positive atom there is some negative electricity, as in every negative atom there is some positive electricity. Thus, in the atoms of copper and zinc positive electricity predominates, said Berzelius, but the zinc atoms are more positive than those of copper; hence, when the metals are brought into contact the negative electricity of the copper atoms is attracted and neutralized by the positive electricity of the zinc atoms, combination takes place, and the compound atom is still characterized by a predominance of positive electricity.

Hence Berzelius identified "electrical polarity" with chemical affinity. Every atom was regarded by him as both positively and negatively electrified; but as one of these electricities was always much stronger than the other, every atom regarded as a whole appeared to be either positively or negatively electrified. Positive atoms showed affinity for negative atoms, and vice versâ. As a positive atom might become more positive by increasing the temperature of the atom, so might the affinity of this atom for that be more marked at high than at low temperatures.

Now, if two elementary atoms unite, the compound atom must—according to the Berzelian views—be characterized either by positive or negative electricity. This compound atom, if positive, will exhibit affinity for other compound atoms in which negative electricity predominates; if negative, it will exhibit affinity for other positively electrified compound atoms. If two compound atoms unite chemically, the complex atom so produced will, again, be characterized by one or other of the two electricities, and as it is positive or negative, so will it exhibit affinity for positively or negatively electrified complex atoms. Thus Berzelius and his followers regarded every compound atom, however complex, as essentially built up of two parts, one of which was positively and the other negatively electrified, and which were held together chemically by virtue of the mutual attractions of these electricities; they regarded every compound atom as a dual structure. The classification adopted by Berzelius was essentially a dualistic classification. His system has always been known in chemistry as dualism.

Berzelius divided compound atoms (we should now say molecules) into three groups or orders—

Compound atoms of the first order, formed by the immediate combination of atoms of two, or in organic compounds of three, elementary substances.

Compound atoms of the second order, formed by the combination of atoms of an element with atoms of the first order, or by the combination of two or more atoms of the first order.

Compound atoms of the third order, formed by combination of two or more atoms of the second order.

When an atom of the third order was decomposed by an electric current, it split up, according to the Berzelian teaching, into atoms of the second order—some positively, others negatively electrified. When an atom of the second order was submitted to electrolysis, it decomposed into atoms of the first order—some positively, others negatively electrified.

Berzelius said that a base is an electro-positive oxide, and an acid is an electro-negative oxide. The more markedly positive an oxide is, the more basic it is; the more negative it is, the more is it characterized by acid properties.

One outcome of this teaching regarding acids and bases was to overthrow the Lavoisierian conception of oxygen as the acidifying element. Some oxides are positive, others negative, said Berzelius; but acids are characterized by negative electricity, therefore the presence of oxygen in a compound does not always confer on that compound acid properties.

We have already seen that silica was regarded by most chemists as a typical earth; but Berzelius found that in the electrolysis of compounds of silica, this substance appeared at the positive pole of the battery—that is, the atom of silica belonged to the negatively electrified order of atoms. Silica was almost certainly an oxide; but electro-negative oxides are, as a class, acids; therefore silica was probably an acid. The supposition of the acid character of silica was amply confirmed by the mineralogical analyses and experiments of Berzelius. He showed that most of the earthy minerals are compounds of silica with electro-positive metallic oxides, and that silica plays the part of an acid in these minerals; and in 1823 he obtained the element silicon, the oxide of which is silica. On this basis Berzelius reared a system of classification in mineralogy which much aided the advance of that branch of natural science.

By the work of Berzelius and Davy the Lavoisierian conception of acid has now been much modified and extended; it has been rendered less rigid, and is therefore more likely than before to be a guide to fresh discoveries.

The older view of acid and alkali was based, for the most part, on a qualitative study of the reactions of chemical substances: bodies were placed in the same class because they were all sour, or all turned vegetable blues to red, etc. This was followed by a closer study of the composition of substances, and by attempts to connect the properties of these substances with their composition; but when this attempt resulted in the promulgation of the dictum that "oxygen is the acidifying principle," it began to be perceived that a larger basis of fact must be laid before just conclusions could be drawn as to the connections between properties and composition of substances. This larger basis was laid by the two chemists whose work we have now reviewed. Of the life of one of these men I have already given such a sketch as I can from the materials available to me; of the life of the other we happily possess ample knowledge. Let us now consider the main features of this life.

Humphry Davy, the eldest son of Robert and Grace Davy, was born at Penzance, in Cornwall, on December 17, 1778, eight months that is before the birth of Berzelius. His parents resided on a small property which had belonged to their ancestors for several generations. Surrounded by many kind friends by whom he was much thought of, the boy appears to have passed a very happy childhood. Even at the age of five his quickness and penetration were marked by those around him, and at school these continued to be his predominant characteristics. Nurtured from his infancy in the midst of beautiful and romantic scenery, and endowed with great observing power and a lively imagination, young Davy seemed destined to be one of those from whose lips is "poured the deathless singing;" all through life he was characterized by a strongly marked poetic temperament.

Humphry Davy was held in much esteem by his school friends as a composer of valentines and love letters, as a daring and entertaining teller of stories, and as a successful fireworks manufacturer. Such a combination of qualities would much endear him to his boy-companions. We are told that at the age of eight he used to mount on an empty cart, around which a circle of boys would collect to be entertained by the wonderful tales of the youthful narrator.

Finishing his school education at the age of fifteen, he now began his own education of himself. In 1795 he was apprenticed to a surgeon and apothecary (afterwards a physician), in Penzance, with whom he learned the elements of medical science; but his time during the years which he spent under Mr. Borlase was much occupied in shooting, fishing, searching for minerals and geological specimens, composing poetry, and pursuing metaphysical speculations. He was now, as through life, an enthusiastic lover of Nature; his mind was extremely active, ranging over the most diverse subjects; he was full of imagination, and seemed certain to distinguish himself in any pursuit to which he should turn his attention. During the next three or four years Davy indulged freely in speculations in all manner of subjects; he started, as people generally do when young, from general principles and followed these out to many conclusions. Even in his study of physiology and other branches of science, he appears at this time to have adopted the speculative rather than the experimental method; but unlike most youthful metaphysicians he was ready to give up an opinion whenever it appeared to him incorrect. By the time he reached the age of twenty he had discarded this method of seeking for truth, and was ever afterwards distinguished by his careful working out of facts as the foundation for all his brilliant theories.

Davy appears to have begun the study of chemistry about 1798 by reading Lavoisier's "Elements of Chemistry," the teachings of which he freely criticized. About this time Mr. Gregory Watt came to live at Penzance as a lodger with Davy's mother, and with him the young philosopher had much talk on chemical and other scientific subjects. He also became acquainted with Mr. Davies Gilbert—who was destined to succeed Davy as President of the Royal Society—and from him he borrowed books and received assistance of various kinds in his studies.

It was during these years that Davy made experiments on heat, which were published some years later, and which are now regarded as laying the foundations of the modern theory according to which heat is due to the motions of the small parts of bodies. He arranged two brass plates so that one should carry a block of ice which might be caused to revolve in contact with the other plate; the plates were covered by a glass jar, from which he exhausted the air by means of a simple syringe of his own contrivance; the machine being placed on blocks of ice the plates were caused to revolve. The ice inside the jar soon melted; Davy concluded that the heat required to melt this ice could only be produced by the friction of the ice and brass, and that therefore heat could not be any form of ponderable matter.

In the year 1798 Davy was asked to go to Bristol as superintendent of the laboratory of a new Pneumatic Institution started by Dr. Beddoes for the application of gases to the treatment of diseases. Davy had corresponded with Beddoes before this time regarding his experiments on heat, and the latter seems to have been struck with his great abilities and to have been anxious to secure him as experimenter for his institution. Davy was released from his engagements with Mr. Borlase, and, now about twenty years of age, set out for his new home, having made as he says all the experiments he could at Penzance, and eagerly looking forward to the better appliances and incitements to research which he hoped to find at Bristol.

The Pneumatic Institution was supported by subscriptions, for the most part from scientific men. It was started on a scientific basis. Researches were to be made on gases of various kinds with the view of applying these as remedies in the alleviation of disease. An hospital for patients, a laboratory for experimental research, and a lecture theatre were provided.

At this time many men of literary and intellectual eminence resided in Bristol; among these were Coleridge and Southey. Most of these men were visitors at the house of Dr. Beddoes, and many distinguished men came from various parts of the county to visit the institution. Davy thus entered on a sphere of labour eminently suited for the development of his genius. With ample mechanical appliances for research, with plenty of time at his disposal, surrounded by an atmosphere of inquiry and by men who would welcome any additions he could make to the knowledge of Nature, and being at the same time not without poetic and imaginative surroundings, by which he was ever spurred onwards in the pursuit of truth—placed in these circumstances, such an enthusiastic and diligent student of science as Davy could not but obtain results of value to his fellows. The state of chemical science at this time was evidently such as to incite the youthful worker. The chains with which Stahl and his successors had so long bound the limbs of the young science had been broken by Lavoisier; and although the French school of chemistry was at this time dominant, and not disinclined to treat as ignorant any persons who might differ from its teaching, yet there was plenty of life in the cultivators of chemistry. The controversy between Berthollet and Proust was about to begin; the Lavoisierian views regarding acids and salts were not altogether accepted by Gay-Lussac, Thenard and others; and from the laboratory of Berzelius there was soon to issue the first of those numerous researches which drew the attention of every chemist to the capital of Sweden. The voltaic battery had been discovered, and had opened up a region of possibilities in chemistry.

Davy began his researches at the institution by experiments with nitrous oxide, a gas supposed by some people at that time to be capable of producing most harmful effects on the animal system. He had to make many experiments before he found a method for preparing the pure gas, and in the course of these experiments he added much to the stock of chemical knowledge regarding the compounds of nitrogen and oxygen. Having obtained fairly pure nitrous oxide, he breathed it from a silk bag; he experienced a "sensation analogous to gentle pressure on all the muscles;... the objects around me became dazzling and my hearing more acute;... at last an irresistible propensity to action was indulged in.... I recollect but indistinctly what followed; I know that my motions were various and violent." Southey and Coleridge breathed the gas; the poets only laughed a little. Encouraged by the results of these experiments, Davy proceeded to prepare and breathe nitric oxide—whereby he was rendered very ill—and then carburetted hydrogen—which nearly killed him.

In his chemical note-book about this time, Davy says, "The perfection of chemical philosophy, or the laws of corpuscular motion, must depend on the knowledge of all the simple substances, their mutual attractions, and the ratio in which the attractions increase or diminish with increase or diminution of temperature.... The first step towards these laws will be the decomposition of those bodies which are at present undecompounded." And in the same note-book he suggests methods which he thinks might effect the decomposition of muriatic and boric acids, the alkalis and earths. Here are the germs of his future work.

After about eight months' work at Bristol he published a volume of "Researches," which contained a great many new facts, and was characterized by vigour and novelty of conception. These researches had been carried out with intense application; each was struck off at a red heat. His mind during this time was filled with vast scientific conceptions, and he began also to think of fame. "An active mind, a deep ideal feeling of good, and a look towards future greatness," he tells us, sustained him.

Count Rumford, the founder of the Royal Institution in London, was anxious to obtain a lecturer on chemistry for the Institution. Davy was strongly recommended, and after a little arrangement—concerning which Davy says in a letter, "I will accept of no appointment except on the sacred terms of independence"—he was appointed Assistant Lecturer on Chemistry and Director of the Laboratory. About a year later his official designation was changed to Professor of Chemistry. This appointment opened up a great sphere of research; "the sole and uncontrolled use of the apparatus of the institution for private experiments" was to be granted him, and he was promised "any apparatus he might need for new experiments."

He had now the command of a good laboratory; he had not to undergo the drudgery of systematic teaching, but was only required to give lectures to a general audience. Before leaving Bristol he had commenced experiments on the chemical applications of the voltaic battery; these he at once followed up with the better apparatus now at his command. The results of this research, and his subsequent work on the alkalis and on muriatic acid and chlorine, have been already described. The circumstances of Davy's life had hitherto been most favourable; how nobly he had availed himself of these circumstances was testified by the work done by him.

His first lecture was delivered in the spring of 1801, and at once he became famous. A friend of Davy says, "The sensation created by his first course of lectures at the Institution, and the enthusiastic admiration which they obtained, is scarcely to be imagined. Men of the first rank and talent, the literary and the scientific, the practical and the theoretical, blue-stockings and women of fashion, the old and the young—all crowded, eagerly crowded the lecture-room. His youth, his simplicity, his natural eloquence, his chemical knowledge, his happy illustrations and well-conducted experiments, excited universal attention and unbounded applause. Compliments, invitations and presents were showered upon him in abundance from all quarters; his society was courted by all, and all appeared proud of his acquaintance." One of his biographers says of these lectures, "He was always in earnest, and when he amused most, amusement appeared most foreign to his object. His great and first object was to instruct, and in conjunction with this, maintain the importance and dignity of science; indeed, the latter, and the kindling a taste for scientific pursuits, might rather be considered his main object, and the conveying instruction a secondary one."

The greatest pains were taken by Davy in the composition and rehearsal of his lectures, and in the arrangement of experiments, that everything should tend towards the enlightenment of his audience. Surrounded by a brilliant society, invited to every fashionable entertainment, flattered by admirers, tempted by hopes of making money, Davy remained a faithful and enthusiastic student of Nature. "I am a lover of Nature," he writes at this time to a friend, "with an ungratified imagination. I shall continue to search for untasted charms, for hidden beauties. My real, my waking existence, is amongst the objects of scientific research. Common amusements and enjoyments are necessary to me only as dreams to interrupt the flow of thoughts too nearly analogous to enlighten and vivify."

During these years (i.e. from 1802 to 1812) he worked for the greater part of each day in the laboratory. Every week, almost every day, saw some fresh discovery of importance. He advanced from discovery to discovery. His work was characterized by that vast industry and extreme rapidity which belong only to the efforts of genius. Never, before or since, has chemical science made such strides in this country.

In 1803 Davy was elected a Fellow, and in 1807 one of the secretaries of the Royal Society. In 1812 he retired from the professorship of chemistry at the Royal Institution; in the same year he was made a knight.

The next two or three years were mostly spent in travelling abroad with his wife—he had married a widow lady, Mrs. Apreece, in 1812. During his visit to Paris he made several experiments on the then recently discovered iodine, and proved this substance to be an element.

The work which Davy had accomplished in the seventeen years that had now elapsed since he began the study of chemistry, whether we consider it simply as a contribution to chemical science, or in the light of the influence it exerted on the researches of others, was of first-rate importance; but a fresh field now began to open before him, from which he was destined to reap the richest fruits. In the autumn of 1815 his attention was drawn to the subject of fire-damp in coal-mines. As he passed through Newcastle, on his return from a holiday spent in the Scottish Highlands, he examined various coal-mines and collected samples of fire-damp; in December of the same year his safety-lamp was perfected, and soon after this it was in the hands of the miner.

The steps in the discovery of this valuable instrument were briefly these. Davy established the fact that fire-damp is a compound of carbon and hydrogen; he found that this gas must be mixed with a large quantity of ordinary air before the mixture becomes explosive, that the temperature at which this explosion occurs is a high one, and that but little heat is produced during the explosion; he found that the explosive mixture could not be fired in narrow metallic tubes, and also that it was rendered non-explosive by addition of carbonic acid or nitrogen. He reasoned on these facts thus: "It occurred to me, as a considerable heat was required for the inflammation of the fire-damp, and as it produced in burning a comparatively small degree of heat, that the effect of carbonic acid and azote, and of the surfaces of small tubes, in preventing its explosion, depended on their cooling powers—upon their lowering the temperature of the exploding mixture so much that it was no longer sufficient for its continuous inflammation." He at once set about constructing a lamp in which it should be impossible for the temperature of ignition of a mixture of fire-damp and air to be attained, and which therefore, while burning, might be filled with this mixture without any danger of an explosion. He surrounded the flame of an oil-lamp with a cylinder of fine wire-gauze; this lamp when brought into an atmosphere containing fire-damp and air could not cause an explosion, because although small explosions might occur in the interior of the wire cylinder, so much heat was conducted away by the large metallic surface that the temperature of the explosive atmosphere outside the lamp could not attain that point at which explosion would occur.

In 1818 Sir Humphry Davy was made a baronet, in recognition of his great services as the inventor of the safety-lamp; and in 1820 he was elected to the most honourable position which can be held by a man of science in this country, he became the President of the Royal Society.

For seven years he was annually re-elected president, and during that time he was the central figure in the scientific society of England. During these years he continued his investigations chiefly on electro-chemical subjects and on various branches of applied science. In 1826 his health began to fail. An attack of paralysis in that year obliged him to relinquish most of his work. He went abroad and travelled in Italy and the Tyrol, sometimes strong enough to shoot or fish a little, or even to carry on electrical experiments; sometimes confined to his room, or to gentle exercise only. He resigned the presidentship of the Royal Society in 1827. In 1828 he visited Rome, where he was again attacked by paralysis, and thought himself dying, but he recovered sufficiently to attempt the journey homeward. At Geneva he became very ill, and expired in that city on the 29th of May 1829.