Heroes of Science: Chemists

But I think if we consider this application of the term "atom" to elements and compounds alike, we shall see objections to it. When an atom of a compound is divided the smaller particles so produced are each very different in chemical properties from the atom which has just been divided. We may, if we choose, assume that the atom of an element could in like manner be divided, and that the products of this division would be different from the elementary atoms; but such a division of an elementary atom has not as a matter of fact been yet accomplished, unless we class among elements substances such as potash and soda, which for many years were universally regarded as elements, and rightly so regarded because they had not been decomposed. In Dalton's nomenclature then, the term "atom" is applied alike to a small particle with definite properties known to be divisible into smaller particles, each with properties different from those of the undivided particle, and to a small particle which, so far as our knowledge goes, cannot be divided into any particle smaller than or different from itself.

Nevertheless, if the atomic theory was to be victorious, it was necessary that it should be applied to elements and compounds alike. Until a clear conception should be obtained, and expressed in accurate language, of the differences in structure of the ultimate particles of compounds and of elements, it was perhaps better to apply the term "atom" to both alike.

These two difficulties—(1) the difficulty of attaching to the term "atom" a precise meaning applicable to elements and compounds alike, and (2) the difficulty of determining the number of elementary atoms in the atom of a given compound, and hence of determining the relative weights of elementary atoms themselves—were for many years stumbling-blocks in the path of the upholders of the Daltonian theory.

The very great difficulty of clearly comprehending the full meaning of Dalton's proposed theory becomes apparent when we learn that within three years from the publication of Part I. of the "New System," facts were made known by the French chemist Gay-Lussac, and the true interpretation of these facts was announced by the Italian chemist Avogadro, which facts and interpretation were sufficient to clear away both the difficulties I have just mentioned; but that nevertheless it is only within the last ten or fifteen years that the true meaning of the facts established by Gay-Lussac and the interpretation given by Avogadro have been generally recognized.

In 1809 Gay-Lussac, in a memoir on the combination of gaseous bodies, proved that gases combine chemically in simple proportions by volume, and that the volume of the product always bears a simple relation to the volumes of the combining gases. Thus, he showed that two volumes of hydrogen combine with one volume of oxygen to form two volumes of water vapour; that one volume of nitrogen combines with three volumes of hydrogen to form two volumes of ammonia gas, and so on. Now, as elements combine atom with atom, the weights of these combining volumes of elements must represent the relative weights of the atoms of the same elements.

In 1811 Avogadro distinguished between the ultimate particles of compounds and elements. Let a gaseous element, A, combine with another gaseous element, B, to form a gaseous compound, C; then Avogadro supposed that the little particles of A and the little particles of B (Dalton's atoms) split up, each into two or more smaller particles, and that these smaller particles then combine together to form particles of the compound C. The smaller particles produced by splitting a Daltonian elementary atom were regarded by Avogadro as all identical in properties, but these very small particles could not exist uncombined either with each other or with very small particles of some other element. When the atom of a compound is decomposed, Avogadro pictured this atom as splitting into smaller particles of two or three or more different kinds, according as the compound had contained two or three or different elements.

To Avogadro's mental vision an elementary gas appeared as built up of a great many little particles, each exhibiting in miniature all the properties of the gas. The gas might be heated, or cooled, or otherwise physically altered, but each of the little particles remained intact; the moment however that this gas was mixed with another on which it could chemically react, these little particles split into smaller parts, but as the smaller parts so produced could not exist in this state, they seized hold of the corresponding very small parts of the other gas, and thus a particle of a compound gas was produced.

A compound gas was pictured by Avogadro as also built up of small particles, each exhibiting in miniature the properties of the gas, and each remaining undecomposed when the gas was subjected only to physical actions; but when the gas was chemically decomposed, each little particle split, but the very small parts thus produced, being each a particle of an elementary substance, continued to exist, and could be recognized by the known properties of that element.

To the smallest particle of any substance (elementary or compound) which exhibits the properties of that substance, and which cannot be split into parts without destroying these properties, we now give the name of molecule.

A molecule is itself a structure. It is built up of parts; each of these parts we now call an atom. The molecule of a compound is, of course, composed of the atoms of the elements which form that compound. The molecule may contain two or three or more unlike atoms. The molecule of an element is composed of the atoms of that element, and all of these atoms are supposed to be alike. We cannot get hold of elementary atoms and examine them, but we have a large mass of evidence in favour of the view which regards the molecule of an element as composed of parts each weighing less than the molecule itself.

The student of physics or chemistry now believes that, were a very small quantity of a gas (say ammonia) or a drop of a liquid (say water) magnified to something like the size of the earth, he should see before him a vast heap of particles of ammonia or of water, each exhibiting all the properties by the possession of which he now distinguishes ammonia or water from all other kinds of matter. He believes that he should see these particles in motion, each moving rapidly from place to place, sometimes knocking against another, sometimes traversing a considerable space without coming into collision with any other. But the student tries to penetrate yet further into the nature of things. To the vision of the chemist these particles of almost inconceivable minuteness are themselves built up of smaller particles. As there is an architecture of masses, so is there an architecture of molecules. Hydrogen and oxygen are mixed; the chemist sees the molecules of each in their never-ceasing dance moving here and there among the molecules of the other, yet each molecule retaining its identity; an electric spark is passed through the mixture, and almost instantaneously he sees each hydrogen molecule split into two parts, and each oxygen molecule split into two parts, and then he sees these parts of molecules, these atoms, combine, a pair of hydrogen atoms with an atom of oxygen, to form compound molecules of water.

Avogadro's hypothesis gave the chemist a definition of "molecule;" it also gave him a definition of "atom."

It is evident that, however many atoms of a given element there may be in this or in that compound molecule, no compound of this element can exist containing less than a single atom of the element in question; therefore an atom of an element is the smallest quantity of that element in the molecule of any compound thereof.

And so we have come back to the original hypothesis of Dalton; but we have extended and modified that hypothesis—we have distinguished two orders of small particles, the molecule (of a compound or of an element) and the atom (of an element). The combination of two or more elements is now regarded as being preceded by the decomposition of the molecules of these elements into atoms. We have defined molecule and we have defined atom, but before we can determine the relative weights of elementary atoms we must have a means of determining the relative weights of compound molecules. The old difficulty still stares us in the face—how can we find the number of elementary atoms in the molecule of a given compound?

The same naturalist who enriched chemical science by the discovery of the molecule as distinct from the atom, placed in the hands of chemists the instrument for determining the relative weights of molecules, and thus also the relative weights of atoms.

The great generalization, usually known as Avogadro's law, runs thus: "Equal volumes of gases measured at the same temperature and under the same pressure contain equal numbers of molecules."

Gay-Lussac had concluded that "equal volumes of gases contain equal numbers of atoms;" but this conclusion was rejected, and rightly rejected by Dalton, who however at the same time refused to admit that there is a simple relation between the combining volumes of elements. The generalization of Avogadro has however stood the test of experiment, and is now accepted as one of the fundamental "laws" of chemical science.

Like the atomic theory itself, Avogadro's law is an outcome of physical work and of physical reasoning. Of late years the great naturalists, Clausius, Helmholtz, Joule, Rankine, Clerk Maxwell and Thomson have developed the physical theory of molecules, and have shown that Avogadro's law may be deduced as a necessary consequence from a few simple physical assumptions. This law has thus been raised, from being a purely empirical generalization, to the rank of a deduction from a wide, yet simple physical theory.

Now, if "equal volumes of gases contain equal numbers of molecules," it follows that the ratio of the densities of any two gases must also be the ratio of the weights of the molecules which constitute these gases. Thus, a given volume of water vapour weighs nine times more than an equal volume of hydrogen; therefore the molecule of gaseous water is nine times heavier than the molecule of hydrogen. One has therefore only to adopt a standard of reference for molecular weights, and Avogadro's law gives the means of determining the number of times any gaseous molecule is heavier than that of the standard molecule.

But consider the combination of a gaseous element with hydrogen; let us take the case of hydrogen and chlorine, which unite to form gaseous hydrochloric acid, and let us determine the volumes of the uniting elements and the volume of the product. Here is a statement of the results: one volume of hydrogen combines with one volume of chlorine to form two volumes of hydrochloric acid. Assume any number of molecules we please in the one volume of hydrogen—say ten—there must be, by Avogadro's law, also ten molecules in the one volume of chlorine; but inasmuch as the volume of hydrochloric acid produced is double that of either the hydrogen or the chlorine which combined to form it, it follows, by the same law, that twenty molecules of hydrochloric acid have been formed by the union of ten molecules of hydrogen with ten molecules of chlorine. The necessary conclusion is that each hydrogen molecule and each chlorine molecule has split into two parts, and that each half-molecule (or atom) of hydrogen has combined with one half-molecule (or atom) of chlorine, to produce one compound molecule of hydrochloric acid.

Therefore we conclude that the hydrogen molecule is composed of two atoms, and that the chlorine molecule is also composed of two atoms; and as hydrogen is to be our standard element, we say that if the atom of hydrogen weighs one, the molecule of the same element weighs two.

It is now easy to find the molecular weight of any gas; it is only necessary to find how many times heavier the given gas is than hydrogen, the weight of the latter being taken as 2. Thus, oxygen is sixteen times heavier than hydrogen, but 1: 16 = 2: 32, therefore the molecule of oxygen is thirty-two times heavier than the molecule of hydrogen. Ammonia is eight and a half times heavier than hydrogen, but 1: 8-1/2 = 2: 17, therefore the molecule of ammonia is seventeen times heavier than the molecule of hydrogen. This is what we more concisely express by saying "the molecular weight of oxygen is 32," or "the molecular weight of ammonia is 17," etc., etc.

Now, we wish to determine the atomic weight of oxygen; that is, we wish to find how many times the oxygen atom is heavier than the atom of hydrogen. We make use of Avogadro's law and of the definition of "atom" which has been deduced from it (see p. 142).

We know that eight parts by weight of oxygen combine with one part by weight of hydrogen to form water; but we do not know whether the molecule of water contains one atom of each element, or two atoms of hydrogen and one atom of oxygen, or some other combination of these atoms (see p. 131). But by vaporizing water and weighing the gas so produced, we find that water vapour is nine times heavier than hydrogen: now, 1: 9 = 2: 18, therefore the molecular weight of water gas is 18. Analysis tells us that eighteen parts by weight of water gas contain sixteen parts of oxygen and two parts of hydrogen; that is to say, we now know that in the molecule of water gas there are two atoms of hydrogen combined with sixteen parts by weight of oxygen. We now proceed to analyze and determine the molecular weights of as many gaseous compounds of oxygen as we can obtain. The outcome of all is that we have as yet failed to obtain any such compound in the molecule of which there are less than sixteen parts by weight of oxygen. In some of these molecules there are sixteen, in some thirty-two, in some forty-eight, in some sixty-four parts by weight of oxygen, but in none is there less than sixteen parts by weight of this element. Therefore we conclude that the atomic weight of oxygen is 16, because this is the smallest amount, referred to hydrogen taken as 1, which has hitherto been found in the molecule of any compound of oxygen.

The whole of the work done since the publication of Dalton's "New System" has emphasized the importance of that chemist's remark, that no safe conclusion can be drawn as to the value of the atomic weight of an element except from a consideration of many compounds of that with other elements. But in Avogadro's law we have a far more accurate and trustworthy method for determining the molecular weights of compounds than any which Dalton was able to devise by his study of chemical combinations.

We have thus got a clearer conception of "atom" than was generally possessed by chemists in the days of Dalton, and this we have gained by introducing the further conception of "molecule" as that of a quantity of matter different from, and yet similar to, the atom.

The task now before us will for the most part consist in tracing the further development of the fundamental conception of Dalton, the conception, viz., of each chemical substance as built up of small parts possessing all the properties, other than the mass, of the whole; and—what we also owe to Dalton—the application of this conception to explain the facts of chemical combination.

The circumstances of Dalton's early life obliged him to trust largely to his own efforts for acquiring knowledge; and his determination not to accept facts at second hand but to acquire them for himself, is very marked throughout the whole of his life. In the preface to the second part of the "New System" he says, "Having been in my progress so often misled by taking for granted the results of others, I have determined to write as little as possible but what I can attest by my own experience."

We should not expect such a man as this to make any great use of books; one of his friends tells us that he heard him declare on a public occasion that he could carry his library on his back, and yet had not read half of the books which comprised it.

The love of investigation which characterized Dalton when young would naturally be increased by this course of intellectual life. How strong this desire to examine everything for himself became, is amusingly illustrated by a story told by his medical adviser, Dr. Ransome. Once when Dalton was suffering from catarrh Dr. Ransome had prescribed a James's powder, and finding his patient much better next day, he congratulated himself and Dalton on the good effects of the medicine. "I do not well see how that can be," said Dalton, "as I kept the powder until I could have an opportunity of analyzing it."

As Dalton grew older he became more than ever disinclined to place much trust in the results obtained by other naturalists, even when these men were acknowledged to be superior to himself in manipulative and experimental skill. Thus, as we have already learned, he could not be brought to allow the truth of Gay-Lussac's experimentally established law regarding gaseous combinations; he preferred to attribute Gay-Lussac's results to errors of experiment. "The truth is, I believe, that gases do not unite in equal or exact measures in any one instance; when they appear to do so it is owing to the inaccuracy of our experiments."

That Dalton did not rank high as an experimenter is evident from the many mistakes in matters of fact which are to be found in the second part of his "New System." A marked example of his inaccuracy in purely experimental work is to be found in the supposed proof given by him that charcoal, after being heated to redness, does not absorb gases. He strongly heated a quantity of charcoal, pulverized it, and placed it in a Florence flask, which was connected by means of a stopcock with a bladder filled with carbonic acid: after a week he found that the flask and its contents had not sensibly increased in weight, and he concluded that no carbonic acid had been absorbed by the charcoal. But no trustworthy result could be obtained from an experiment in which the charcoal, having been deprived of air by heating, was again allowed to absorb air by being pulverized in an open vessel, and was then placed in a flask filled with air, communication between the carbonic acid and the external air being prevented merely by a piece of bladder, a material which is easily permeated by gases.

Dalton used a method which can only lead to notable results in natural science when employed by a really great thinker; he acquired a few facts, and then thought out the meaning of these. Almost at the beginning of each investigation he tried to get hold of some definite generalization, and then he proceeded to amass special facts. The object which he kept before himself in his experimental work was to establish or to disprove this or that hypothesis. Every experiment was conducted with a clearly conceived aim. He was even willing to allow a large margin for errors of experiment if he could thereby bring the results within the scope of his hypothesis.

That the law of multiple proportions is simply a generalization of facts, and may be stated apart from the atomic theory, is now generally admitted. But in Dalton's mind this law seems to have arisen rather as a deduction from the theory of atoms than to have been gained as a generalization from experiments. He certainly always stated this law in the language of the atomic theory. In one of his walking excursions he explained his theory to a friend, and after expounding his views regarding atomic combinations, he said that the examples which he had given showed the necessary existence of the principle of multiple proportions: "Thou knowest it must be so, for no man can split an atom." We have seen that carburetted hydrogen was one of the compounds on the results of the analysis of which he built his atomic theory; yet we find him saying of the constitution of this compound that "no correct notion seems to have been formed till the atomic theory was introduced and applied in the investigation."

When Dalton was meditating on the laws of chemical combination, a French chemist, M. Proust, published analyses of metallic oxides, which proved that when a metal forms two oxides the amount of metal in each is a fixed quantity—that there is a sudden jump, as it were, from one oxide to another. We are sometimes told that from these experiments Proust would have recognized the law of multiple proportions had his analyses only been more accurate; but we know that Dalton's analyses were very inaccurate, and yet he not only recognized the law of multiple proportions, but propounded and established the atomic theory. Something more than a correct system of keeping books and balancing accounts is wanted in natural science. Dalton's experimental results would be the despair of a systematic analyst, but from these Dalton's genius evolved that splendid theory which has done so much to advance the exact investigation of natural phenomena.

Probably no greater contrast could be found between methods of work, both leading to the establishment of scientific (that is, accurate and precise) results, than that which exists between the method of Dalton and the method pursued by Priestley.

Priestley commenced his experiments with no particular aim in view; sometimes he wanted to amuse himself, sometimes he thought he might light upon a discovery of importance, sometimes his curiosity incited him to experiment. When he got facts he made no profound generalizations; he was content to interpret his results by the help of the prevailing theory of his time. But each new fact only spurred him on to make fresh incursions into the fields of Nature. Dalton thought much and deeply; his experimentally established facts were to him symbols of unseen powers. He used facts as Hobbes says the wise man uses words: they were his counters only, not his money.

When we ask how it was that Dalton acquired his great power of penetrating beneath the surface of things and finding general laws, we must attribute this power in part to the training which he gave himself in physical science. It was from a consideration of physical facts that he gained the conception of ultimate particles of definite weight. His method was essentially dynamical; that is, he pictured a gas as a mass of little particles, each of which acted on and was acted on by, other particles. The particles were not thrown together anyhow; definite forces existed between them. Each elementary or compound gas was pictured as a system of little particles, and the properties of that gas were regarded as dependent on the nature and arrangement of these particles. Such a conception as this could only be gained by a careful and profound thinker versed in the methods of physical and mathematical science. Thus we see that although Dalton appeared to gain his great chemical results by a method which we are not generally inclined to regard as the method of natural science, yet it was by virtue of his careful training in a branch of knowledge which deals with facts, as well as in that science which deduces particular conclusions from general principles, that he was able to introduce his fruitful conceptions into the science of chemistry.

To me it appears that Dalton was pre-eminently distinguished by the possession of imagination. He formed clear mental images of the phenomena which he studied, and these images he was able to combine and modify so that there resulted a new image containing in itself all the essential parts of each separate picture which he had previously formed.

From his intense devotion to the pursuit of science the development of Dalton's general character appears to have been somewhat dwarfed. Although he possessed imagination, it was the imagination of a naturalist rather than that of a man of broad culture. Perhaps it was a want of broad sympathies which made him trust so implicitly in his own work and so readily distrust the work of others, and which moreover led him astray in so many of his purely experimental investigations.

Dalton began his chemical work about six years after the death of Lavoisier. Unlike that great philosopher he cared nothing for political life. The friends in whose family he spent the greater part of his life in Manchester were never able to tell whether he was Whig or Tory. Unlike Priestley he was content to let metaphysical and theological speculation alone. In his quiet devotion to study he more resembled Black, and in his method, which was more deductive than that usually employed in chemistry, he also resembled the Edinburgh professor. Trained from his earliest days to depend on himself, nurtured in the creedless creed of the Friends, he entered on his life's work with few prejudices, if without much profound knowledge of what had been done before him. By the power of his insight into Nature and the concentration of his thought, he drew aside the curtain which hung between the seen and the unseen; and while Herschel, sweeping the heavens with his telescope and night by night bringing new worlds within the sphere of knowledge, was overpowering men's minds by new conceptions of the infinitely great, John Dalton, with like imaginative power, was examining the architecture of the ultimate particles of matter, and revealing the existence of law and order in the domain of the infinitely small.

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.

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—