CHAPTER IV. ESTABLISHMENT OF GENERAL PRINCIPLES OF CHEMICAL SCIENCE ( continued )--PERIOD OF DAVY AND BERZELIUS. Humphry Davy , 1778-1829. Johann Jacob Berzelius , 1779-1848.

We may roughly date the period of chemical advance during which the connections between chemistry and other branches of natural knowledge were recognized and studied, as beginning with the first year of this century, and as continuing to our own day.

The elaboration of the atomic theory was busily carried on during the second and third decades of this century; to this the labour of the Swedish chemist Berzelius largely contributed.

That there exist many points of close connection between chemical and electrical science was also demonstrated by the labours of the same chemist, and by the brilliant and impressive discoveries of Sir Humphry Davy.

A system of classification of chemical elements and compounds was established by the same great naturalists, and many inroads were made into the domain of the chemistry of bodies of animal and vegetable origin.

The work of Berzelius and Davy, characterized as it is by thoroughness, clearness and definiteness, belongs essentially to the modern era of chemical advance; but I think we shall better preserve the continuity of our story if we devote a chapter to a consideration of the work of these two renowned naturalists before entering on our review of the time immediately preceding the present, as typical workers in which time I have chosen Liebig and Dumas.

In the last chapter we found that the foundations of the atomic theory had been laid, and the theory itself had been applied to general problems of chemical synthesis, by Dalton. In giving, in that chapter, a short sketch of the modern molecular theory, and in trying to explain the meaning of the term "molecule" as contrasted with "atom," I necessarily carried the reader forward to a time considerably later than the first decade of this century. We must now retrace our steps; and in perusing the account of the work of Berzelius and Davy given in the present chapter, the reader must endeavour to have in his mind a conception of atom analogous to the mental picture formed by Dalton (see pp. 135, 136); he must regard the term as applicable to element and compound alike; he must remember that the work of which he reads is the work of those who are striving towards a clear conception of the atom, and who are gradually rising to a recognition of the existence of more than one order of small particles, by the regular putting together of which masses of matter are constituted.

No materials, so far as I am aware, exist from which a life of Berzelius can be constructed. I must therefore content myself with giving a mere enumeration of the more salient points in his life. Of his chemical work abundant details are fortunately to be found in his own "Lehrbuch," and in the works and papers of himself and his contemporaries.

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Johann Jacob Berzelius was the son of the schoolmaster of WÄfersunda, a village near LinkÖping, in East Gothland, Sweden. He was born in August 1779—he was born, that is, a few years after Priestley's discovery of oxygen; at the time when Lavoisier had nearly completed his theory of combustion; when Dalton was endeavouring to keep the unruly youth of Eaglesfield in subjection; and when Black, having established the existence of fixed air and the theory of latent heat, was the central figure in the band of students who were enlarging our knowledge of Nature in the Scottish capital.

Being left an orphan at the age of nine, the young Berzelius was brought up by his grandfather, who appears to have been a man of education and sense. After attending school at LinkÖping, he entered the University of Upsala as a student of medicine. Here he soon began to show a taste for chemistry. It would appear that few or no experiments were then introduced into his lectures by the Professor of Chemistry at Upsala; little encouragement was given to pursue chemical experiments, and so Berzelius had to trust to his own labours for gaining an acquaintance with practical chemistry. Having thus made considerable progress in chemistry, and being on a visit to the mineral baths of Medevi, he seized the opportunity to make a very thorough analysis of the waters of this place, which were renowned in Sweden for their curative properties. The publication of this analysis marks the first appearance of Berzelius as an author.

He graduated as M.B. in 1801, and a year or two later presented his dissertation, entitled "The Action of Galvanism on Organic Bodies," as a thesis for the degree of Doctor of Medicine. This thesis, like that of Black, published about half a century earlier, marks an important stage in the history of chemistry. These and other publications made the young doctor famous; he was called to Stockholm to be extraordinary (or assistant) Professor of Chemistry in the medical school of that capital.

Sometimes practising medicine in order to add to his limited income, but for the most part engaged in chemical research, he remained in Stockholm for nearly fifty years, during most of which time the laboratory of Berzelius in the Swedish capital was regarded as one of the magnetic poles of the chemical world. To this point came many of the great chemists who afterwards enriched the science by their discoveries. WÖhler, H. and G. Rose, Magnus, Gmelin, Mitscherlich and others all studied with Berzelius. He visited England and France, and was on terms of intimacy and in correspondence with Davy, Dalton, Gay-Lussac, Berthollet and the other men who at that period shed so much lustre on English and French science.

It is said that Berzelius was so much pleased with the lectures of Dr. Marcet at Guy's Hospital, that on his return from his visit to England in 1812, he introduced much more liveliness and many more experimental illustrations into his own lectures.

At the age of thirty-one, Berzelius was chosen President of the Stockholm Academy of Sciences; a few years later he was elected a Foreign Fellow of the Royal Society, which society bestowed on him the Copley Medal in 1836. He was raised to the rank of a baron by the King of Sweden, being allowed as a special privilege to retain his own name.

In the year 1832 Berzelius resigned his professorship, and in the same year he married. During the remainder of his life, he continued to receive honours of all kinds, but he never for a moment forsook the paths of science. After the death of Davy, in 1829, he was recognized as the leading European chemist of his age; but, although firm in his own theoretical views, he was ready to test these views by appealing to Nature. The very persistency with which he clung to a conception established on some solid experimental basis insured that new light would be thrown on that conception by the researches of those chemists who opposed him.

Probably no chemist has added to the science so many carefully determined facts as Berzelius; he was always at work in the laboratory, and always worked with the greatest care. Yet the appliances at his command were what we should now call poor, meagre, and utterly inadequate. Professor WÖhler of GÖttingen, who in the fulness of days and honours has so lately gone from amongst us, recently gave an account of his visit to Berzelius in the year 1823. WÖhler had taken his degree as Doctor of Medicine at Heidelberg, and being anxious to prosecute the study of chemistry he was advised by his friends to spend a winter in the laboratory of the Swedish professor. Having written to Berzelius and learned that he was willing to allow him working room in his laboratory, the young student set out for Stockholm. After a journey to LÜbeck and a few days' passage in a small sailing-vessel, he arrived in the Swedish capital.

Knocking at the door of the house pointed out as that of Berzelius, he tells us that his heart beat hard as the door was opened by a tall man of florid complexion. "It was Berzelius himself," he exclaims. Scarcely believing that he was in the very room where so many famous discoveries had been made, he entered the laboratory. No water, no gas, no draught-places, no ovens were to be seen; a couple of plain tables, a blowpipe, a few shelves with bottles, a little simple apparatus, and a large water-barrel whereat Anna, the ancient cook of the establishment, washed the laboratory dishes, completed the furnishings of this room, famous throughout Europe for the work which had been done in it. In the kitchen which adjoined, and where Anna cooked, was a small furnace and a sand bath for heating purposes.

In this room many great discoveries were made. Among these we may note the separation of the element columbium in 1815, and of selenion in 1818; the discovery of the new earth thoria in 1828; the elucidation of the properties of yttrium and cerium about 1820, of uranium in 1823, and of the platinum metals in 1828; the accurate determination of the atomic weights of the greater number of the elements; the discovery of "sulphur salts" in 1826-27, and the proof that silica is an acid, and that most of the "stony" minerals are compounds of this acid with various bases.

But we shall better learn the value of some of these discoveries by taking a general review of the contributions to chemical science of the man who spent most of his life at work in that room in Stockholm.

The German chemist Richter, in the first or second year of this century, had drawn attention to the fact that when two neutral compounds, such as nitrate of potash and chloride of lime, react chemically, the substances produced by this reaction are also neutral. All the potash combined with nitric acid in one salt changes places with all the lime combined with muriatic acid in the other salt; therefore, said Richter, these different quantities of potash and lime are neutralized by the same quantity of nitric acid; and, hence, these amounts of potash and lime are chemically equivalent, because these are the amounts which perform the same reaction, viz. neutralization of a fixed quantity of acid. If then careful analyses were made of a number of such neutral compounds as those named, the equivalents of all the commoner "bases" and "acids"[9] might be calculated.

Richter's own determinations of the equivalents of acids and bases were not very accurate, but Berzelius was impressed with the importance of this work. The year before the appearance of Dalton's "New System" (i.e. in 1807), he began to prepare and carefully analyze series of neutral salts. As the work was proceeding he became acquainted with the theory of Dalton, and at once saw its extreme importance. For some time Berzelius continued to work on the lines laid down by Dalton, and to accumulate data from which the atomic weights of elements might be calculated; but he soon perceived—as the founder of the theory had perceived from the very outset—that the fundamental conception of each atom of an element as being a distinct mass of matter weighing more or less than the atom of every other element, and of each atom of a compound as being built up of the atoms of the elements which compose that compound,—Berzelius, I say, perceived that these conceptions must remain fruitless unless means were found for determining the number of elementary atoms in each compound atom. We have already learned the rules framed by the founder of the atomic theory for his guidance in attempting to solve this problem. Berzelius thought those rules insufficient and arbitrary; he therefore laid down two general rules, on the lines of which he prosecuted his researches into chemical synthesis.

"One atom of one element combines with one, two, three, or more atoms of another element." This is practically the same as Dalton's definitions of binary, ternary, etc., compounds (p. 132). "Two atoms of one element combine with three and five atoms of another element." Berzelius here recognizes the existence of compound atoms of a more complex structure than any of those recognized by Dalton.

Berzelius further extended the conception of atom by applying it to groups of elements formed, according to him, by the combination of various compound atoms. To his mind every compound atom appeared as built up of two parts; each of these parts might be an elementary atom, or might be itself built up of several elementary atoms, yet in the Berzelian theory each acted as a definite whole. So far as the building up of the complex atom went, each of the two parts into which this atom could be divided acted as if it were a simple atom.

If we suppose a patch of two shades of red colour to be laid on a smooth surface, and alongside of this a patch of two shades of yellow colour, and if we suppose the whole mass of colour to be viewed from a distance such that one patch appears uniformly red and the other uniformly yellow, we shall have a rough illustration of the Berzelian compound atom. To the observer the whole mass of colour appears to consist of two distinct patches of contrasted colours; but let him approach nearer, and he perceives that what appeared to be a uniform surface of red or yellow really consists of two patches of unlike shades of red or of yellow. The whole mass of colour represents the compound atom; broadly it consists of two parts—the red colour represents one of the constituent atoms, the yellow colour represents the other constituent atom; but on closer examination the red atom, so to speak—and likewise the yellow atom—is found to consist of parts which are less unlike each other than the whole red atom is unlike the whole yellow atom.

We shall have to consider in more detail the reasoning whereby Berzelius arrived at this conception of every compound atom as a dual structure (see pp. 209-212). At present I wish to notice this conception as lying at the root of most of the work which he did in extending and applying the Daltonian theory. I wish to insist on the fact that the atomic theory could not advance without methods being found for determining the number of elementary atoms in a compound atom, without clear conceptions being gained of every compound atom as a structure, and without at least attempts being made to learn the laws in accordance with which that structure was built. Before the atomic weight of oxygen could be determined it was necessary that the number of oxygen and of hydrogen atoms in the atom of water should be known; otherwise all that could be stated was, the atomic weight of oxygen is a simple multiple of 8. Berzelius did much to advance chemical science by the introduction and application of a few simple rules whereby he determined the number of elementary atoms in various compound atoms. But as the science advanced, and as more facts came to be known, the Berzelian rules were found to be too narrow and too arbitrary; chemists sought for some surer and more generally applicable method than that which Berzelius had introduced, and the imperious demand for this method at last forced them to recognize the importance of the great generalization of the Italian naturalist Avogadro, which they had possessed since the year 1811, but the meaning of which they had so long failed to understand.

Berzelius made one great step in the direction of recognizing Avogadro's distinction between atom and molecule when he accepted Gay-Lussac's generalization that "equal volumes of gases contain equal numbers of atoms:" but he refused to apply this to other than elementary gases. The weights of the volumes of elementary gases which combined were, for Berzelius, also the weights of the atoms of these elements. Thus, let the weight of one volume of hydrogen be called 1, then two volumes of hydrogen, weighing 2, combine with one volume of oxygen, weighing 16, to form two volumes of water vapour; therefore, said Berzelius, the atom of water consists of two atoms of hydrogen and one atom of oxygen, and the atom of the latter element is sixteen times heavier than the atom of the former. Three volumes of hydrogen, weighing 3, combine with one volume of nitrogen, weighing 14, to form two volumes of ammonia; therefore, said Berzelius, the atom of ammonia consists of three atoms of hydrogen combined with one atom of nitrogen, and the nitrogen atom is fourteen times heavier than the atom of hydrogen.

While Berzelius was applying these rules to the determination of the atomic weights of the elements, and was conducting the most important series of analyses known in the annals of the science, two great physico-chemical discoveries were announced.

In the year 1818 the "law of isomorphism" was stated by Mitscherlich: "Compounds the atoms of which contain equal numbers of elementary atoms, similarly arranged, have the same crystalline form." As thus stated, the law of isomorphism affirms that if two compounds crystallize in the same form, the atoms of these compounds are built up of the same number of elementary atoms—however different may be the nature of the elements in the compounds—and that these elementary atoms are similarly arranged. This statement was soon found to be too absolute, and was accordingly modified; but to go into the history of the law of isomorphism would lead us too far from the great main path of chemical advance, the course of which we are seeking to trace.

Berzelius at once accepted Mitscherlich's law, as an aid in his researches on atomic weights. The help to be derived from this law may be illustrated thus: let us assume that two compounds have been obtained exhibiting identity of crystalline form; let it be further assumed that the number of elementary atoms in the atom of one of these compounds is known; it follows, by the law of isomorphism, that the number of elementary atoms in the atom of the other is known also. Let the two compounds be sulphate of potash and chromate of potash; let it be assumed that the atom of the first named is known to consist of two atoms of potassium, one atom of sulphur, and four atoms of oxygen; and that the second substance is known to be a compound of the elements potassium, chromium and oxygen; then the atom of the second compound contains, by Mitscherlich's law, two atoms of potassium, one atom of chromium and four atoms of oxygen: hence the relative weight of the atom of chromate of potash can be determined, and hence the relative weight of the atom of chromium can also be determined.

A year after the announcement of Mitscherlich's law, the following generalization was stated to hold good, by two French naturalists, Dulong and Petit:—"The atoms of all solid elements have the same capacity for heat."

If the amount of heat required to raise the temperature of one grain of water through one degree be called one unit of heat, then the capacity for heat of any body other than water is the number of units of heat required to raise the temperature of one grain of that substance through one degree. Each chemical substance, elementary and compound, has its own capacity for heat; but, instead of comparing the capacities for heat of equal weights, Dulong and Petit compared the capacities for heat of weights representing the weights of the atoms of various elements. Thus, equal amounts of heat are required to raise, through the same interval of temperature, fifty-six grains of iron, one hundred and eight grains of silver, and sixty-three and a half grains of copper; but the weights of the atoms of these three elements are in the proportion of 56:108:63-1/2. Dulong and Petit based their generalization on measurements of the capacities for heat of thirteen elements; further research has shown that their statement most probably holds good for all the solid elements. Here then was a most important instrument put into the hands of the chemist.

It is only necessary that the atomic weight of one solid element should be certainly known, and that the amount of heat required to raise through one degree the number of grains of that element expressed by its atomic weight should also be known; then the number which expresses the weight, in grains, of any other solid element which is raised through one degree by the same amount of heat, likewise expresses the relative weight of the atom of that element. Thus, suppose that the atomic weight of silver is known to be 108, and suppose that six units of heat are required to raise the temperature of one hundred and eight grains of this metal through one degree; then suppose it is found by experiment that six units of heat suffice to raise the temperature of two hundred and ten grains of bismuth through one degree, it follows—according to the law of Dulong and Petit—that 210 is the atomic weight of bismuth.

The modified generalization of Gay-Lussac—"Equal volumes of elementary gases contain equal numbers of atoms;" the laws of "isomorphism" and of "atomic heat;" and the two empirical rules stated on p. 163;—these were the guides used by Berzelius in interpreting the analytical results which he and his pupils obtained in that memorable series of researches, whereby the conceptions of Dalton were shown to be applicable to a wide range of chemical phenomena.

The fixity of composition of chemical compounds has now been established; a definite meaning has been given to the term "element;" the conception of "atom" has been gained, but much remains to be done in the way of rendering this conception precise; and fairly good, but not altogether satisfactory methods have been introduced by which the relative weights of the atoms of elements and compounds may be determined. At this time chemists are busy preparing and describing new compounds, and many new elements are also being discovered; the need of classification begins to be felt more and more.

In the days of Berzelius and Davy strenuous efforts were made to obtain some generalizations by the application of which the many known elements and compounds might be divided into groups. It was felt that a classification might be founded on the composition of compounds, or perhaps on the properties of the same compounds. These two general principles served as guides in most of the researches then instituted; answers were sought to these two questions: Of what elements is this compound composed? and, What can this compound do; how does it react towards other bodies?

Lavoisier, as we know, regarded oxygen as the characteristic element of all acids. This term acid implies the possession, by all the substances denoted by it, of some common property; let us shortly trace the history of this word in chemistry.

Vinegar was known to the Greeks and Romans, and the names which they gave this substance tell us that sourness was to them its characteristic property. They knew that vinegar effervesced when brought into contact with chalky earths, and that it was able to dissolve many substances—witness the story of Cleopatra's draught of the pearl dissolved in vinegar. Other substances possessed of these properties—for instance oil of vitriol and spirits of salt—as they became known, were classed along with vinegar; but no attempts were made to clearly define the properties of these bodies till comparatively recent times.

The characteristics of an acid substance enumerated by Boyle are—solvent power, which is exerted unequally on different bodies; power of turning many vegetable blues to red, and of restoring many vegetable colours which had been destroyed by alkalis; power of precipitating solid sulphur from solutions of this substance in alkalis, and the power of acting on alkalis to produce substances without the properties of either acid or alkali.

But what, one may ask, is an alkali, of which mention is so often made by Boyle?

From very early times it had been noticed that the ashes which remained when certain plants were burned, and the liquid obtained by dissolving those ashes in water, had great cleansing powers; that they removed oily matter, fat and dirt from cloth and other fabrics. The fact that an aqueous solution of these ashes affects the coloured parts of many plants was also noticed in early times. As progress was made in chemical knowledge observers began to contrast the properties of this plant-ash with the properties of acids. The former had no marked taste, the latter were always very sour; the former turned some vegetable reds to blue, the latter turned the blues to red; a solution of plant-ash had no great solvent action on ordinary mineral matter, whereas this matter was generally dissolved by an acid. In the time of the alchemists, who were always seeking for the principles or essences of things, these properties of acids were attributed to a principle of acidity, while the properties of plant-ash and substances resembling plant-ash were attributed to a principle of alkalinity (from Arabic alkali, or the ash).

In the seventeenth century the distinction between acid and alkali was made the basis of a system of chemical medicine. The two principles of acidity and alkalinity were regarded as engaged in an active and never-ending warfare. Every disease was traced to an undue preponderance of one or other of these principles; to keep these unruly principles in quietness became the aim of the physician, and of course it was necessary that the physician should be a chemist, in order that he might know the nature and habits of the principles which gave him so much trouble.

Up to this time the term "alkali" had been applied to almost any substance having the properties which I have just enumerated; but this group of substances was divided by Van Helmont and his successors into fixed alkali and volatile alkali, and fixed alkali was further subdivided into mineral alkali (what we now call soda) and vegetable alkali (potash). About the same time acids were likewise divided into three groups; vegetable, animal, and mineral acids. To the properties by which alkali was distinguished, viz. cleansing power and action on vegetable colouring matters, Stahl (the founder of the phlogistic theory) added that of combining with acids. When an acid (that is, a sour-tasting substance which dissolves most earthy matters and turns vegetable blues to red) is added to an alkali (that is, a substance which feels soap-like to the touch, which does not dissolve many earthy matters, and which turns many vegetable reds to blue) the properties of both acid and alkali disappear, and a new substance is produced which is not characterized by the properties of either constituent. The new substance, as a rule, is without action on earthy matters or on vegetable colours; it is not sour, nor is it soapy to the touch like alkali; it is neutral. It is a salt. But, although Stahl stated that an alkali is a substance which combines with an acid, it was not until a century later that these three—alkali, acid, salt—were clearly distinguished.

But the knowledge that a certain group of bodies are sour and dissolve minerals, etc., and that a certain other group of bodies are nearly tasteless and do not dissolve minerals, etc., was evidently a knowledge of only the outlying properties of the bodies; it simply enabled a term to be applied to a group of bodies, which term had a definite connotation.

Why are acids acid, and why are alkalis alkaline?

Acids are acid, said Becher (latter part of seventeenth century), because they all contain the same principle, viz. the primordial acid. This primordial acid is more or less mixed with earthy matter in all actual acids; it is very pure in spirits of salt.

Alkalis are alkaline, said Basil Valentine (beginning of the sixteenth century), because they contain a special kind of matter, "the matter of fire."

According to other chemists (e.g. J. F. Meyer, 1764), acids owe their acidity to the presence of a sharp or biting principle got from fire.

Acids, alkalis and salts all contain, according to Stahl (beginning of the eighteenth century), more or less primordial acid. The more of this a substance contains, the more acid it is; the less of this it contains, the more alkaline it is.

All these attempted explanations recognize that similar properties are to be traced to similarity of composition; but the assertion of the existence of a "primordial acid," or of "the matter of fire," although undoubtedly a step in advance, was not sufficiently definite (unless it was supplemented by a distinct account of the properties of these principles) to be accepted when chemical knowledge became accurate.

The same general consideration, founded on a large accumulation of facts, viz. that similarity of properties is due to similarity of composition, guided Lavoisier in his work on acids. He found the "primordial acid" of Stahl, and the "biting principle" of Meyer, in the element oxygen.

I have already (p. 91) shortly traced the reasoning whereby Lavoisier arrived at the conclusion that oxygen is the acid-producer; here I would insist on the difference between his method and that of Basil Valentine, Stahl and the older chemists. They carried into the domain of natural science conceptions obtained from, and essentially belonging to the domain of metaphysical or extra-physical speculation; he said that oxygen is the acidifier, because all the compounds of this element which he actually examined were possessed of the properties included under the name acid. We know that Lavoisier's conclusion was erroneous, that it was not founded on a sufficiently broad basis of facts. The conception of an acidifying principle, although that principle was identified with a known element, was still tainted with the vices of the alchemical school. We shall see immediately how much harm was done by the assertion of Lavoisier, "All acids contain oxygen."

In Chapter II. (pp. 32-37) we traced the progress of knowledge regarding alkalis from the time when the properties of these bodies were said to be due to the existence in them of "matter of fire," to the time when Black had clearly distinguished and defined caustic alkali and carbonated alkali.

The truly philosophical character, and at the same time the want of enthusiasm, of Black become apparent if we contrast his work on alkali with that of Lavoisier on acid. Black did not hamper the advance of chemistry by finding a "principle of alkalinity;" but neither did he give a full explanation of the fact that certain bodies are alkaline while others are not. He set himself the problem of accurately determining the differences in composition between burnt (or caustic) and unburnt (or mild) alkali, and he solved the problem most successfully. He showed that the properties of mild alkalis differ from those of caustic alkalis, because the composition of the former differs from that of the latter; and he showed exactly wherein this difference of composition consists, viz. in the possession or non-possession of fixed air.

Strange we may say that this discovery did not induce Black to prosecute the study of caustic alkalis: surely he would have anticipated Davy, and have been known as the discoverer of potassium and sodium.

In the time of Stahl the name "salt" was applied, as we have learned, to the substance produced by the union of an acid with an alkali; but the same word was used by the alchemists with an altogether different signification. Originally applied to the solid matter obtained by boiling down sea-water, and then extended to include all substances which, like this solid matter, are very easily dissolved by water and can be recovered by boiling down this solution, "salt" was, in the sixteenth and seventeenth centuries, the name given to one of the hypothetical principles or elements. Many kinds of matter were known to be easily dissolved by water; the common possession of these properties was sought to be accounted for by saying that all these substances contained the same principle, namely, the principle of salt. I have already tried to indicate the reasoning whereby Boyle did so much to overthrow this conception of salt. He also extended our knowledge of special substances which are now classed as salts. The chemists who came after Boyle gradually reverted to the older meaning of the term "salt," adopting as the characteristics of all substances placed in this class, ready solubility in water, fusibility, or sometimes volatility, and the possession of a taste more or less like that of sea-salt.

Substances which resembled salts in general appearance, but were insoluble in water, and very fixed in the fire, were called "earths"; and, as was generally done in those days, the existence of a primordial earth was assumed, more or less of which was supposed to be present in actual earths. This recognition of the possibility of more or less of the primordial earth being present in actually occurring earths, of course necessitated the existence of various kinds of earth. The earths were gradually distinguished from each other; lime was recognized as a substance distinct from baryta, baryta as distinct from alumina, etc.

Stahl taught that one essential property of an earth was fusibility by fire, with production of a substance more or less like glass. This property was possessed in a remarkable degree by quartz or silica. Hence silica was regarded as the typical earth, until Berzelius, in 1815, proved it to be an acid. But the earths resembled alkalis, inasmuch as they too combined with, and so neutralized, acids.

There is an alkali hidden in every earth, said some chemists.

An alkali is an earth refined by the presence of acid and combustible matter, said others.

Earths thus came to be included in the term "alkali," when that term was used in its widest acceptation. But a little later it was found that some of the earths were thrown down in the solid form from their solutions in acids by the addition of alkalis; this led to a threefold division, thus—

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.

During these later years of illness and suffering, his intense love of and delight in Nature were very apparent; he returned again to the simple tastes and pleasures of his early days. His intimate knowledge of natural appearances and of the sights and sounds of country life is conspicuous in the "Salmonia, or Days of Fly-fishing," written during his later years.

Sir Humphry Davy was emphatically a genius. He was full of eager desire to know the secrets of the world in which he lived; he looked around him with wonder and delight, ever conscious of the vastness of the appearances which met his gaze; an exuberance of life and energy marked his actions; difficulties were encountered by him only to be overcome; he was depressed by no misfortunes, deterred by no obstacles, led aside from his object by no temptations, and held in bondage by no false analogies.

His work must ever remain as a model to the student of science. A thorough and careful foundation of fact is laid; on this, hypotheses are raised, to be tested first by reasoning and argument, then by the tests of the laboratory, which alone are final. Analogies are seized; hints are eagerly taken up, examined, and acted on or dismissed. As he works in the laboratory, we see his mind ranging over the whole field of chemical knowledge, finding a solution of a difficulty here, or guessing at a solution there; combining apparently most diverse facts; examining phenomena which appear to have no connection; never dwelling too long on an hypothesis which cannot yield some clue to the object of research, but quickly discovering the road which will lead to the wished-for solution.

Like so many great experimenters Davy accomplished wonders with little apparatus. When he went abroad for the first time he took with him two small boxes, one twenty, and the other twelve inches long, by about seven inches wide and four deep. With the apparatus contained in these boxes he established the elementary nature of iodine, and made a rough estimation of its atomic weight; he determined many of its analogies with chlorine, proving that, like chlorine, it is markedly electro-negative, and that its compounds are decomposed by chlorine; he accomplished the synthesis of hydriodic acid, and approximately determined the composition of iodide of nitrogen. But when it was necessary to employ delicate or powerful apparatus, he was able by the use of that also to obtain results of primary importance. The decomposition of potash, soda, baryta, lime and strontia could not have been effected had he not had at his command the resources of a well-furnished laboratory.

Davy has had no successor in England. Much useful and some brilliant work has been done by English chemists since his day, but we still look back to the first quarter of the century as the golden age of chemistry in this country. On the roll wherein are written the names of England's greatest sons, there is inscribed but a single chemist—Humphry Davy.

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I carried on the account of the work of Davy's great contemporary, Berzelius, to the time when he had fairly established dualistic views of the structure of chemical compounds, and when, by the application of a few simple rules regarding the combinations of elementary atoms, he had largely extended the bounds of the atomic theory of Dalton.

Berzelius also did important work in the domain of organic chemistry. By numerous analyses of compounds of animal and vegetable origin, he clearly established the fact that the same laws of combination, the same fixity of composition, and the same general features of atomic structure prevail among the so-called organic as among the inorganic compounds. In doing this he broke down the artificial barrier which had been raised between the two branches of the science, and so prepared the way for modern chemistry, which has won its chief triumphs in the examination of organic compounds.

By the many and great improvements which he introduced into analytical chemistry, and by the publication of his "Textbook of Chemistry," which went through several editions in French and German, and also of his yearly report on the advance of chemistry, Berzelius exerted a great influence on the progress of his favourite science. WÖhler tells us that when the spring of the year came, at which time his annual report had to be prepared, Berzelius shut himself up in his study, surrounded himself with books, and did not stir from the writing-table until the work was done.

In his later days Berzelius was much engaged in controversy with the leaders of the new school, the rise and progress of which will be traced in the next chapter, but throughout this controversy he found time to add many fresh facts to those already known. He continued his researches until his death in 1848.

The work of the great Swedish chemist is characterized by thoroughness in all its parts: to him every fact appeared to be of importance; although now perhaps only an isolated fact, he saw that some day it would find a place in a general scheme of classification. He worked in great measure on the lines laid down by Dalton and Davy; the enormous number and accuracy of his analyses established the law of multiple proportions on a sure basis, and his attempts to determine the constitution of compound atoms, while advancing the atomic theory of Dalton, drew attention to the all-important distinction between atom and molecule, and so prepared chemists for the acceptance of the generalization of Avogadro. The electro-chemical conceptions of Davy were modified by Berzelius; they were shorn of something of their elasticity, but were rendered more suited to be the basis of a rigid theory.

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At the close of this transition period from the Lavoisierian to the modern chemistry, we find analytical chemistry established as an art; we find the atomic theory generally accepted, but we notice the existence of much confusion which has arisen from the non-acceptance of the distinction made by Avogadro between atom and molecule; we find the analogies between chemical affinity and electrical energy made the basis of a system of classification which regards every compound atom (or molecule) as built up of two parts, in one of which positive, and in the other negative electricity predominates; and accompanying this system of classification we find that an acid is no longer regarded as necessarily an oxygen compound, but rather as a compound possessed of certain properties which are probably due to the arrangement of the elementary atoms, among which hydrogen appears generally to find a place; we find that salts are for the most part regarded as metallic derivatives of acids; and we find that by the decomposition of the supposed elementary substances, potash, soda, lime, etc., the number of the elements has been extended, the application of a new instrument of research has been brilliantly rewarded, and the Lavoisierian description of "element" as the "attained, not the attainable, limit of research" has been emphasized.

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