Popular Science Monthly/Volume 59/June 1901/The Periodic Law
| THE PERIODIC LAW. |
By Professor JAS. LEWIS HOWE,
WASHINGTON AND LEE UNIVERSITY.
BEFORE the time of Lavoisier ideas concerning the nature of matter were mere speculations. Following the introduction, at the close of the eighteenth century, of the conception of the indestructibility of matter, and more especially with the introduction of the atomic theory a decade or so later, the idea of some sixty or seventy absolutely different kinds of matter received general acceptance. The unity of these different elements was, indeed, held by some, but as a pure speculation, while the evidence was all against it. It remained for the Periodic Law to show that there is a connection between these different elements. It is true, we are as far as ever from any knowledge of what that connection is, or from any knowledge of the nature of that primal substance out of which all matter is shaped, unless, indeed, the recent work of J. J. Thomson and others on the electric condition of gases is pointing us thitherward.
The early attempts to classify substances from a chemical, or rather alchemistical standpoint, were wholly superficial. Pliny, for example, describes two forms of lead, plumbum nigrum and plumbum candidum. The former term was used for lead proper, the latter for tin, though these two metals have little resemblance, except in their low melting points. Sulfuric acid was classed with the oils, as oil of vitriol, and the name has popularly and technically remained to the present, although the only resemblance of sulfuric acid to an oil is in its appearance. The chlorids of antimony and of tin were known respectively as butter of antimony and butter of tin, from the fact that they are semisolid substances, of much the same consistency as butter from milk. Even to-day we speak familiarly of milk of lime and milk of sulfur, though but for the fact that they are whitish liquids, they have nothing in common with the product from the cow. Perhaps to us one of the most remarkable instances of classification was the association of the black oxid of manganese with the white oxid of magnesium, commonly known as calcined magnesia. The only property common to these two, magnesia nigra and magnesia alba, as they were early called, is that both are fine powders. In the seventeenth and eighteenth centuries the discovery of the different gases began, and to the workers of that day all were but different kinds of air. Thus we find 'inflammable air* as the name for hydrogen, 'fixed air* for carbonic acid gas, and 'dephlogisticated marine acid air' for chlorin. That no better principle of classification of substances from a chemical standpoint than that of superficial outward appearance was demanded is not strange, when we recollect that at this period and, indeed, down to the close of the eighteenth century, the transmutation of metals was a popular belief of the common people and was not disproved by the chemist. There was no underlying, unchangeable principle at the basis of the different substances with which the chemist had to deal.
Lavoisier—government medallist at twenty-one, adjunct member of the French Academy at twenty-five, chemist, geologist, mineralogist and mathematician, man of business and amasser of wealth, financier, reformer, fermier general, imprisoned by Robespierre on the trumpedup charge of having adulterated tobacco with water, guillotined in 1794, when only just past fifty years old—this is the man whom the French, with much justice, call the 'Father of Chemistry,' the man who made chemistry possible as a science by furnishing it with a foundation, the doctrine of the indestructibility of matter. This he accomplished by the use of the balance. A familiar experiment had often been used to support the old idea of transmutation. When water has been boiled for a long time in a glass vessel, on evaporating the water an earthy residue is obtained, and this, said the chemists of that day, is conclusive evidence that water can be transmuted into earth by boiling; and, if water into earth, why not other substances; and why not, if we only knew the method, even the base metals into gold? When less than thirty years old, Lavoisier repeated this experiment, but he took the precaution of weighing his glass vessel with its contained water. After a hundred days' boiling, he found that there was no change in weight. On then evaporating the water he found, indeed, an earthy residue, but the glass vessel had lost an amount exactly equal in weight to that recovered from the water. In other words, the water, so far from being changed into earth, had merely dissolved out a small portion of the glass container. This and many other similar experiments the keen-witted Frenchman used to prove the indestructibility of matter, and on this fundamental doctrine the superstructure of scientific chemistry began to rise.
With this doctrine established, it became possible to consider the nature of matter from a new standpoint, and to define with some accuracy a chemical element. Back in the days of Greek philosophers, elements were very variously conceived of. To Pherekides earth was the primal element; to Anaximenes, air; to Herakleitos, fire; while Thales found in water the first principle of all things, and the followers of the Milesian philosopher were not a few for more than two millenniums. Empedokles accepted all four of these elements, and to them Aristotle added a fifth, ether, the quintessence, subtler and more divine than the other four. With these the alchemists placed a number of substances, approaching somewhat our present idea of elements, but even down to Lavoisier's time the old Greek conceptions were not abandoned. Lavoisier's definition of an element deserves to be quoted, since more than a century of chemical progress has failed to improve or in any essential way change it. "An element," says he, in his 'Traité de Chimie,' "is a substance from which no simpler body has as yet been obtained; a body in which no change causes a diminution of weight. Every substance is to be regarded as an element until it is proved to be otherwise." With his conception of an element, Lavoisier introduced a new and scientific nomenclature into chemistry, which is to a very considerable extent in use to-day. The views of Lavoisier did not gain immediate recognition, but a decade after his untimely death the new ideas had been very generally adopted.
With the opening of a new century came the rehabilitation of a theory which had originated back in the misty days of early Greek philosophy, but which was now to be given a new value, because no longer a vague guess, but founded upon experimental evidence. This was the theory of the atomic constitution of matter. According to the Greek conception, if matter were divided into smaller and ever smaller portions, at last a point would be reached where the particles are indivisible, and such particles are the atoms. Dalton seems first to have hazarded the idea of atoms, almost as a speculation, to account (wrongly) for the various different degrees of solubility of different gases in water, and at this early stage he published a table of familiar substances, with the atomic weight of each. A decided confirmation was given to this guess by Dalton's discovery that when different gases combine, it is always in proportions expressed by whole numbers. This could most readily be explained by the theory that these gases were made up of indivisible particles called atoms, whose union conditioned the proportion between the uniting masses of gases. This atomic theory was somewhat combated by a few chemists, even Sir Humphry Davy and Mr. Wollaston for a brief time opposing it. It soon, however, made its way, and its general principles have been received by all chemists; for nearly a century it has dominated, or rather has been the foundation of, chemical theory.
According to this theory, all matter is composed of some seventy different kinds of atoms, each possessed of independent and permanent properties. Lists of the atomic weights of the commoner elements were rapidly published, the most notable being that of Berzelius, in 1815.
The fact of so many different kinds of ultimate particles of matter was naturally a great blow to those who believed in its unity. Very early there was speculation as to whether any connection existed between the different kinds of atoms. The first step in this direction was what is known as Front's hypothesis, which was first enunciated in 1815, with no author's signature, in Thomson's 'Annals of Philosophy.' Prout had noticed that the 'atomic weights of many of the lighter elements seemed to be exact multiples of that of hydrogen; hence he made the suggestion that all the different atoms may be merely aggregations of the simple hydrogen atom, and that this hydrogen atom is really the primitive element from which all other substances are made.
This was the first attempt to determine a relation between the apparently different kinds of matter, and it was more than eighty years before the advocates of the theory were finally forced to abandon it. There have been few laws in chemistry, and certainly no false hypotheses, which have given rise to so much investigation as that which has been occasioned by Prout's hypothesis. That it could have 60 long retained adherents among chemists, many of them men of great prominence, is due to the fact that it seems on its face to be true. When it was first published, a very considerable number of the atomic weights were approximately multiples of the weight of the hydrogen atom, far more than could be accounted for by chance. It seemed reasonable to believe that, with the meager facilities for accurate work at that day, the atoms of the few other elements would prove, when they should be accurately determined, to be also exact multiples of the hydrogen atom. This view was held by many chemists until a Belgian chemist, Jean Servais Stas, undertook to determine the atomic weight of a few of the elements with an accuracy far greater than had been known up to that time. Prout's hypothesis had been sustained by rounding off the decimals to whole numbers; Stas, before he began this work an earnest believer in the hypothesis, endeavored to determine at least one place of decimals so accurately that it could not hereafter be neglected, and his work is one of the classics of chemistry. He proved clearly that the atoms of several elements, at least, could not be multiples of that of hydrogen. Some of the supporters of the hypothesis then assumed that it was not the hydrogen atom, but a half of it, or some other fraction, which is the original matter, from which all other atoms are derived. The hypothesis may be said to have finally ended its long career when Professor Morley, of Adelbert College, showed that there is no simple ratio between the atomic weights of oxygen and hydrogen; that, instead of being 16:1, it is 15.879:1. For accuracy Professor Morley's work may be justly compared with that of Stas, but in conception of experiment and in difficulty of execution it far surpasses that of the Belgian chemist.
But while it may be considered as absolutely proved that a large share of the atoms have weights which are not exact multiples of that of hydrogen, yet it remains true that many of those which have been determined with the greatest degree of certainty do approach with wonderful closeness to exact multiples. This is shown by the following table, taken from the report of the Committee on Atomic Weights of the American Chemical Society for 1899:
| Arsenic | 75 | .0 | Lead | 206 | .92 | Phosphorus | 31 | .0 |
| Boron | 11 | .0 | Lithium | 7 | .03 | Rhodium | 103 | .0 |
| Bromin | 79 | .95 | Manganese | 55 | .0 | Silver | 107 | .92 |
| Carbon | 12 | .0 | Mercury | 200 | .0 | Sodium | 23 | .05 |
| Cerium | 139 | .0 | Nitrogen | 14 | .04 | Sulfur | 32 | .07 |
| Cobalt | 59 | .00 | Osmium | 191 | .0 | Tin | 119 | .0 |
| Gallium | 70 | .0 | Oxygen | 16 | .00 | Yttrium | 89 | .0 |
| Iron | 56 | .0 | Palladium | 107 | .0 |
In addition to these at least nine others have atomic weights differing not more than 0.1 from whole numbers. By the law of probabilities this close approach to whole numbers cannot be the result of chance, but no satisfactory explanation has as yet been offered.
Prout's hypothesis was not unique in concerning itself with an effort to show a unity of matter. Very early there was noticed a connection between the atomic weights and the properties of certain groups of elements. Attention was first called to this by Professor Döbereiner, of Jena, and an account was given of it in print in 1816, just after the first enunciation of Prout's hypothesis. Döbereiner noticed that the equivalent weight of strontium was 50, while the values then accepted for calcium and barium were respectively 27.5 and 72.5. Fifty is the mean of 27.5 and 72.5, and the properties of strontium may be looked upon as being an average of those of calcium and barium. It was soon clear, however, that strontium was as much entitled to recognition as an element as calcium or barium. Hence it appeared that there was a numerical relation between the weights of the atoms of these three elements, barium, strontium and calcium, which corresponded to both the chemical and the physical properties of the elements. Several other similar groups of three were discovered by Döbereiner, and this, which was really the earliest germ of the Periodic Law, became known as Döbereiner's law of triads. It is of especial interest, as having enabled its author to predict the atomic weight of bromin, which was later confirmed by experimental investigation. In this respect it anticipated the Periodic Law, and may be said to represent a phase of this later and greater generalization.
Little attention was attracted by the speculations of Döbereiner, and a quarter of a century or so later the subject was taken up anew by the great French chemist, Dumas. He developed to some extent the law of triads, though he made little actual advance beyond the point attained by Döbereiner. Dumas's work was, however, widely noticed, and proved very stimulating to the chemists of his day. It is interesting to read the comments of Faraday: "This circumstance (the numerical relations between chlorin, bromin and iodin) has been made the basis of some beautiful speculations by M. Dumas, speculations which have scarcely yet assumed the consistence of a theory, and which are at the present time to be ranged among the poetic daydreams of a philosopher; to be regarded as some of the poetic illuminations of the mental horizon, which possibly may be the harbinger of a new law. . . . We seem here to have the dawning of a new light, indicative of the mutual convertibility of certain groups of elements, although under conditions which as yet are hidden from our scrutiny." In the succeeding decade we find many chemists speculating in a similar way upon the connection which seemed to subsist between the different elements.
The two chemists whose names are associated with the dawn of the Periodic Law are De Chancourtois and Newlands. De Chancourtois arranged the elements in the order of their atomic weights in a helix inscribed upon a vertical cylinder; this he called a 'telluric screw,' and although there were many inaccuracies, as a whole it approached a form in which the Periodic Law is to-day sometimes represented. The ideas of De Chancourtois were by no means free from considerable haze, as, for example, when he states that 'the properties of bodies are the properties of numbers.' This may well be interpreted in the light of the Periodic Law, which affirms that the properties of elements are functions of their atomic weights. Even the important idea of periodicity is not overlooked by De Chancourtois, but the speculations of this ingenious French engineer and geologist had practically no effect upon the chemical thought of that day; indeed, his articles were almost unnoticed and were resurrected only after they had slumbered for nearly thirty years in obscurity.
Somewhat otherwise was it with the work of Newlands, which began to appear in 1863, just a year later than that of his French contemporary. His work was, however, wholly independent of that of De Chancourtois. His first paper was chiefly concerned with the development of numerical relations between the atomic weights, following out the ideas early expressed in Döbereiner's triads. He enlarged this so as to include more than three elements in a group. For example, not only was sodium the middle member, with mean properties, of the triad, lithium, sodium, potassium; but rubidium also belonged to this group, because two of potassium plus one of lithium gives the atomic weight of rubidium. A year later he announced his law of octaves, which is generally looked upon as a forerunner of the Periodic Law. Here he arranged the elements in the order of their atomic weights and showed that "elements having consecutive numbers frequently either belong to the same group or occupy similar positions in different groups." "The difference between the number of the lowest member of a group and that immediately above it is seven; in other words, the eighth element starting from a given one, is a kind of repetition of the first, like the eighth note of an octave in music." While this regularity appeared in the case of the elements of low atomic weight, it failed when applied to many of those elements which have a higher weight, and also in the case of iron, cobalt and nickel. These three metals seem to break in upon the octaves, and must be left out of account before the law of octaves can be used. This irregularity Newlands noticed, enunciating his law in the words: "The numbers of analogous elements, when not consecutive, differ by seven, or by some multiple of seven." By 'number' he means merely the number of the element when all are arranged in a series in the order of their atomic weight. There was thus here, as in the work of De Chancourtois, the vision of a certain periodicity in the actual arrangement of the elements, and the recognition of the fact that in some way there is a connection between the properties of an element and its atomic weight. But there seemed to be no suspicion that all the properties of an element are a function, much less that they are a periodic function of its atomic weight.
It may seem strange to us that the work of these two pioneers should have been received with almost complete indifference by chemists. This results in part, at least, as has been pointed out by Mendeléeff, from the too-limited application of Newlands's law. Relations were brought out between little groups of elements, like Döbereiner's triads, but comparisons were not made between dissimilar elements, and even the groups made up by taking the seventh elements often contained those which were far from being similar in their properties. Thus we find the first, and hence analogous, elements of his eight octaves as follows: Hydrogen, fluorin, chlorin, cobalt-nickel, bromin, palladium, tellurium, platinum-iridium. Such a grouping as this could hardly be expected to appeal strongly to chemists, especially as iodin, an element which obviously belongs with fluorin, chlorin and bromin, is relegated to the seventh group of elements. The lack of enthusiasm on the part of chemists at the reception of Newlands's work may be judged from an incident. When his paper was read at the meeting of the Chemical Society, one of the members present asked of Professor Newlands whether he had ever tried arranging the elements according to the order of their initial letters.
The Periodic Law in its present form was first enunciated by Professor Dmitri Mendelèeff, in 1869, in a paper read before the Russian Physico-Chemical Society. It is true that five years earlier Lothair Meyer had published in the first edition of his 'Modern Theory of Chemistry' a list of the elements arranged according to atomic weights. somewhat after the method of Newlands, but this table could be considered in no sense an advance upon the table of his English contemporary. It was not so much a periodic table as a summary of the grouping of the elements in more or less natural groups. Meyer, indeed, made his earlier table the basis of his later work, but these subsequent amendments to the table were made after the publication of Mendeléeff's first table and show clearly the influence of his work.
In his first paper, Mendeléeff; gives several different arrangements of the elements, all, however, embodying the same principles. The principal table, of which the others are variants, shows many errors and crudities, but the underlying principles of the Periodic Law, as to-day recognized, are clearly apparent. This table is as follows:
MENDELÉEFF'S FIRST TABLE. 1809.
The resemblance to the modern tables comes out yet more strongly when we examine Mendeléeff's horizontal table, which was published in the same paper, and in which the doubtful elements and those whose position was not clear were omitted:
MENDELÉEFF'S HORIZONTAL TABLE. 1868.
In the first table the elements are arranged in vertical columns, in the order of their atomic weights. They fall in a way into groups of seven, as in Newlands's octaves, but after the first two octaves there are quite a number of elements before the next group of seven is reached, and the same is true in each succeeding column. The next year, 1870, Meyer published a table in which he brought these outside elements into something of order by pointing out the existence of a double periodicity after the first two octaves have been passed, and showing that the alternate periods resemble each other closely. This was brought out with greater clearness in the revised table which Mendeléeff published in 1871. The skill of the author of this table is apparent when we consider that it is, with few additions, the generally accepted table in use at the present day. This table, which is given on the following page, when compared with that of two years before, shows how great had been the development.
One well-recognized test of the truth of any theory is its use in prediction. In this table Mendeléeff did not hesitate to make certain changes in the generally received atomic weights, in order to bring facts into conformity with his table. His was not the position of the ancient philosopher who would have all phenomena bend to his preconceived theory, and if the facts failed to yield, so much the worse for the facts. Mendeléeff had confidence that this Periodic Law was the expression of a great truth of nature, and so firm was his confidence that he could not but believe that when the phenomena did not agree, it was from imperfect observation and interpretation of the facts. A good instance of this is seen in the case of the metals of the platinum group. As far as observation had gone, osmium had the largest atomic weight of these metals, followed by iridium and platinum, of equal weight, and all these metals were lighter than gold. According to the Periodic Law the reverse should be the case. Mendeléeff affirmed that the discrepancy in this case was probably due to the fact that the atomic weights of these metals had not been determined with an accuracy commensurate with the work of the table. It is an interesting confirmation that some years later, Seubert took up this atomic weight problem, and found that the views of Mendeléeff were correct. Gold has the highest atomic weight of these elements, platinum the next highest, iridium follows and osmium comes lowest of all, its previously determined weight having been seven or eight units too high.
Along the line of predictions a still more remarkable use of the table appeared in connection with the vacant spaces. There were many places in the table where elements might be expected, which were, however, then unknown. Could the table stand the test of actually predicting the existence of an unknown element? Mendeléeff did not think this too great a strain to put upon his work, and he ventured not merely to predict that elements might be expected with atomic weights of 44, 68 and 72, but he was even bold enough to describe these elements under the names of eka-boron, eka-aluminum and mendeleeff's table of 1871.
eka-silicon, in several cases going into considerable detail as to the properties of the elements and their compounds. It was in 1875 that the first of these predictions was fulfilled in the discovery of gallium by Lecoq de Boisbaudran. This metal fell in the place which Mendeléeff had given to eka-aluminum, and its specific gravity is 5.9, while 5.8 was the figure which had been foretold. Four years later Nilson discovered eka-boron, and gave to it the name scandium. In 1885 a new silver mineral, argyrodite, was found in the Freiberg mines, and every analysis made of it showed a discrepancy of six or seven per cent. This soon led to the recognition by the analyst, Clemens Winkler, of the presence of a new element, and it further appeared that this new element was Mendeléeff's eka-silicon. Not to be outdone by the French and Swedish chemists, Winkler patriotically called the new metal germanium. It is worth while to show side by side, a few of the predictions of the properties of eka-silicon, published by Mendeléeff in 1872, and the actual properties of germanium, as experimentally determined by Winkler in 1886:
| Eka-Silicon. Symbol, Es. | Germanium. Symbol, Ge. |
| ELEMENT. | |
| Atomic weight, 72. | Atomic weight, 72.3. |
| Specific gravity, 5.5. | Specific gravity, 5.469 at 20°. |
| OXID. | |
| Formula, EsO2. | Formula, GeO2 |
| Specific gravity, 4.7. | Specific gravity, 4. 703 at 18°. |
| CHLORID. | |
| Formula, EsCl4. | Formula, GeCl4. |
| Liquid, boiling a little below 100°. | Liquid, boiling at 86°. |
| Specific gravity, 1.9 at 0°. | Specific gravity, 1.887 at 18°. |
| METALLO-ORGANIC COMPOUND. | |
| Formula, Es (C2H5)4. | Formula, Ge (C2H5)4. |
| Liquid, boiling point, 160°. | Liquid, boiling point, 160°. |
| Specific gravity, 0.96. | Specific gravity, slightly less than water (which is 1.0). |
So close is this agreement that it is difficult to realize that Mendeléeff's forecasts were put in print more than a decade before the element had ever been handled by man.
Since the corrected form of Mendeléeff's table was published in 1871, there has been no end to the speculation upon the subject, and dozens of tables, emphasizing different relations of the elements, have been proposed. Few of these have equaled that of the Russian chemist in simplicity or have as few obscure points. One of these tables, suggested a few years ago by Dr. F. P. Venable, of the University of North Carolina, may be noticed as presenting some decided advantages over that of Mendeléeff: VENABLE'S TABLE. 1895. (SLIGHTLY MODIFIED.)
Possible elements, now unknown. Eka-Manganese.
Most tables present a difficulty in that they place sodium in the group or sub-group with copper, silver and gold, while it would most naturally fall in the group with lithium, potassium, rubidium and cesium. So magnesium would be placed according to its properties, not so closely with zinc and cadmium, as with glucinum, calcium, strontium and barium. Fluorin belongs rather with chlorin, bromin and iodin than with manganese, the metal with which it is associated in most tables. So oxygen belongs with sulfur, selenium and tellurium, rather than with chromium and molybdenum. This is remedied in Venable's table. The first element in any group the author calls the group element or bridge element; for it often possesses properties which ally it to the elements of the next groups. The second element he calls the type element, and in this is, as it were, shadowed forth the character of the succeeding elements of the same group. From this point on, the group is divided into two series, one more and the other less electro-positive or negative, as the case may be. The first three groups are for the most part made up of electro-positive elements, while those of the fifth, sixth and seventh groups are relatively electronegative. Now in the former the elements of the more positive series resemble the type element of their group more strongly than those of the less positive series. In the relatively negative fifth, sixth and seventh groups, the reverse is the case. This grouping thus places sodium with potassium and chlorin with bromin. In the fourth group, which lies between the extremes, the type element, silicon, foreshadows the members of its two series to an approximately equal extent.
As the tables which graphically portray the Periodic Law stand to-day, there is much which still remains to be cleared up. At the very outset we are met by the fact that we cannot tell in which group as familiar an element as hydrogen ought to be placed. It generally receives the first place in group one, but to some extent at least this position is based upon a misapprehension. Some years ago Pictet, of Geneva, was engaged in that work on the condensation and liquefaction of gases, which has rendered his name famous. On compressing hydrogen at a very low temperature, he obtained, on suddenly reducing the pressure, some heavy, steel-blue drops, much resembling mercury. This was erroneously supposed to be hydrogen in the liquid form. As a result, it seemed only natural to classify hydrogen with the metallic elements of the first group. Not only have later investigations shown that these drops were not liquid hydrogen, but quite recently Dewar has actually obtained this substance, which proves to be a colorless, limpid liquid, with the extraordinarily low specific gravity of about 0.07. As far then as physical properties go, there is no justification in classifying hydrogen with the metals of group one. Chemically, however, hydrogen is like the metals, electro-positive, though very weakly so, and it is possible that its position in the first group is less awkward than would be any other.
Of the other elements in the table with atomic weight below 100, all seem fairly well placed, though we have not as much knowledge of scandium as we could wish, and there is a difficulty that we shall soon notice in the eighth group. The first apparent blank space in the table is for an element with atomic weight of about 100. Such an element would be known as eka-manganese, and would possess properties which would to a considerable extent resemble those of manganese, but perhaps more closely those of ruthenium. Beyond this in the table we find many gaps, partly from the inadequacy of our chemical knowledge and partly from the likelihood that there exist rare elements which have not yet been discovered. Such elements probably occur in extremely small quantities, and may, for many years, perhaps forever, elude chemists. It seems improbable that there are undiscovered elements which exist in more than very small quantities; this is the testimony, not only of the chemical laboratory, but also of the spectroscope, that instrument which reveals to us the composition, not merely of substances in the laboratory, but also that of the sun and of the distant stars. From barium to tantalum few elements are known to which a definite place can be assigned, but here there is an indefinitely large number of what Crookes has called meta-clements, the rare earths. Their relation to the table is as yet hardly more than speculative, and they have been likened to the asteroids of the solar system.
From tantalum to bismuth the table is very regular, except that the element of the manganese series, which would have an atomic weight of about 188, is unknown, as we have already seen is the case with eka-manganese. Above bismuth, with its weight of 208, we know but two elements, thorium, 831, and uranium, 240. Since the discovery of the Rontgen rays, great interest has been excited by different kinds of rays which, though they may not be visible to the naked eye, are nevertheless capable of affecting the photographic plate. The only elements, as far as yet known, which yield such rays are these two of extraordinarily high atomic weight, thorium and uranium. Closely connected with this phenomenon is that of giving off luminous rays when not exposed to light. It has recently been discovered that while uranium can give off comparatively feeble rays, there is contained in the principal uranium mineral, pitchblende, matter which is much more active than uranium. Further investigation seems to show that there are at least three such substances present in pitchblende; which have been named radium (from the rays it gives off), polonium (from its discoverer's native land), and actinium (from its ratio-activity). Of these radium alone has been studied at all extensively, and even its claim to be called a chemical element is by no means established. It strongly resembles barium, but it gives off rays easily visible in the dark, continuing to shine indefinitely. There is much doubt as to whether it be not really a peculiar form of barium, but recent determinations of its atomic weight, in a condition only partially purified, indicate that it has a higher atomic weight than barium, and that it may in this respect resemble thorium and uranium.
It may seem rather remarkable that, inasmuch as Döbereiner had brought out the resemblances between elements of the same group in his triads, nearly half a century should have elapsed before the essential features of the Periodic Law were discovered. This is due, chiefly at least, to three causes. First, there would have been many more gaps in the table then than now, so many new elements having been discovered since that day; second, the atomic weights of the elements were then so imperfectly known that, using the weights then accepted, it is impossible to construct a periodic table; the third great difficulty lay in the fact that nine very important elements refused to be reduced to order, and finally were excluded and relegated to an outlying group of unique properties. These nine elements are iron, cobalt, nickel and the so-called platinum metals, platinum, palladium, iridium, rhodium, osmium and ruthenium. As a matter of fact, these nine metals cannot be brought into any of the seven regular groups, but must be placed by themselves in a single group of three series, or in three groups. Thia eighth group proves to be transitional between group seven and group one; iron, cobalt and nickel make a direct gradation from manganese to copper; ruthenium, rhodium and palladium, from molybdenum to silver; osmium, iridium and platinum, from tungsten to gold. Not only do these three triplets stand between these other elements in atomic weight, but their properties also show a similar gradation.
While now we have these transitional elements, the question might very naturally arise whether there are similar transitional elements from fluorin to sodium and from chlorin to potassium. The case here is, however, somewhat different from the former one. Manganese and copper are both metals, and not so widely separated in properties; the transitional elements, iron, cobalt and nickel, partake of the nature of both extremes, and the transition seems a natural one. Hardly any elements can be more unlike than fluorin and sodium, or chlorin and potassium. Chlorin is very electro-negative, potassium as strongly electro-positive. A transitional element would thus probably be inert, that is, lacking in both electro-positiveness and in electro-negativeness, and up to a few years ago such an element could hardly have been conceived of. At that time Lord Rayleigh was engaged in determining with all possible accuracy the density of nitrogen. In this work he prepared nitrogen by several different methods. Some specimens were obtained by the decomposition of chemical compounds, such as urea and ammonium nitrite, others from the air by removing the oxygen. To his surprise. Lord Rayleigh found that in every case the nitrogen obtained from the atmosphere was slightly heavier than that prepared from chemical compounds. In searching for the cause of this difference. Lord Rayleigh and Professor Ramsay, who had been associated with him in this work, found that there is present in the atmosphere a new gas, much like nitrogen in its properties, whose existence, although it is present to the extent of nearly one per cent, had been unsuspected. This gas, christened argon from its inertness, is nearly three times as heavy as nitrogen, and it is this that increases the weight of atmospheric nitrogen slightly above the weight of pure nitrogen, obtained from chemical compounds. Stimulated by this discovery it was not long before Ramsay had isolated from the atmosphere at least two other gases, both characterized by an inertness similar to that of argon. These are helium, whose spectrum had long been known from the fact that this gas is plentiful in the corona of the sun, and neon. It is probable that there are several other similar gases in the atmosphere, and one, xenon, has been recently isolated by Ramsay. It is not uninteresting to note that argon had been in the hands of chemists from the time of Cavendish down, but all had supposed it to be nitrogen. Under the influence of the electric spark oxygen and nitrogen may be made to combine with each other. In Cavendish's experiment the spark was passed through the air, which consists chiefly of a mixture of nitrogen and oxygen, and the resultant oxid of nitrogen was absorbed in caustic potash. More oxygen was added from time to time until the last of the nitrogen was used up. Now Cavendish noticed, as have many chemists since his day, that it was always impossible to remove all the nitrogen; in every case about one per cent, of gas remained. There is no record that any one ever suspected that this residue was not nitrogen; such is, however, the fact, and the gases, argon, helium and, perhaps, others are present. These can be best recognized by passing an electric spark through the rarefied gas and examining the spectrum. It seems now very strange to us that an element so abundant that an ordinary sized room contains no less than a thousand liters should have so long escaped discovery. The reason is not far to seek. Argon and its congeners are distinguished by a most remarkable exhibition of properties, in that they have apparently no chemical affinity, and no compounds of them are known. From this fact it has been argued by some that these gases cannot be considered chemical elements, for all elements hitherto known do form compounds with other elements. It is, however, a curious fact that in the periodic table we find, in the eighth group, place for several just such elements, as we have seen, without affinity, and neither positive nor negative in electrochemical character. It may well be that helium, neon, argon and xenon belong in these vacant spaces.
If this be the case, there is still a difficulty which confronts us, and this is that argon possesses an atomic weight slightly higher than the next element in order, potassium, instead of lower. This would not, however, be a unique instance of such a difficulty in the table. It was formerly thought that the two metals, nickel and cobalt, had identical atomic weights, and though the salts of nickel are generally green, and those of cobalt red, in other respects these metals and their compounds are very much alike. After the discovery of the Periodic Law, when it was seen that cobalt belonged in the second series of the eighth group and nickel in the third, it was supposed that further study would necessarily show nickel to have an appreciably higher atomic weight than cobalt. We have already seen that in this same group, before the appearance of the periodic table, the accepted atomic weights of osmium, iridium and platinum were incorrect, and it was the fact of their mis-arrangement in the table which caused Seubert to revise their weights. Very much labor has been spent upon the revision of the atomic weights of nickel and cobalt. Gerhardt Krüss supposed he had found a new and heavier metal, hitherto unknown, in ordinary cobalt, and that this caused the atomic weight to be estimated too high. He called the new metal gnomium, but it was soon shown that gnomium has no real existence. The more accurate the determinations, the more probable it seemed that in reality cobalt, and not nickel, as demanded by theory, has the greater weight. The whole subject has been very carefully investigated in the last few years by Prof. Theodore W. Richards at Harvard University, and there seems now to be no doubt but that Nature has unexpectedly and inexplicably reversed the position of these elements.
Another instance that seems to be of the same nature is that as far as the most accurate determinations go, tellurium has an atomic weight greater than that of iodin, instead of less. At present it is impossible to explain these abnormalities, but they assure us of the possibility that argon may have a higher atomic weight than potassium, and yet belong to the eighth group.
What now is the present position of the philosophy of matter from the light thrown upon it by the Periodic Law? In the first place, drawing our deduction from the marvelously accurate determinations of. the relative weights of the atoms of the different elements, to which chemists have been incited by the Periodic Law, it may be considered as absolutely settled that the elements are not groups of hydrogen atoms, nor are they composed of half or quarter hydrogen atoms. As enunciated by Prout, the hypothesis which goes by his name may be considered as finally proved untenable; the atomic weights are not multiples of the weight of the hydrogen atom, nor any simple fraction thereof. But while this is the case, it is perfectly clear from the Periodic Law that the properties of an atom are a periodic function of its atomic weight. It would seem that this can be true only if the material of which all atoms are made is the same. This does not necessarily mean that there is but one kind of matter, and that all atoms are merely different quantities of this 'urstoff.' There may be several kinds of matter, and different kinds of atoms may represent varying proportions of a few constituents.
There have been many attempts to reduce the Periodic Law to mathematics, in order to find a numerical value for the function which expresses the relation between atomic weight and an element's place in the series. Such efforts have been thus far wholly unsuccessful. It is by no means impossible that such relations will be found in the future, but at present the atomic weights of comparatively few elements have been determined with great accuracy. When this work has been extended to a greater number of elements, and when the position of the rare elements and of the inert atmospheric gases has been definitely settled, we may hope for more light upon the principles underlying the Periodic Law.
At present this law occupies much the same position as two other great generalizations of natural science. The fact of gravitation was long ago discovered. The laws by which it acts are well known, and yet the cause of gravitation is even to-day a mere speculation, and no link has yet been discovered to connect the three phases of attraction, gravitation between masses, cohesion and adhesion between molecules, and chemical affinity between atoms. The fact of evolution is universally recognized, much is known and more is a matter of speculation as to how and why evolution has taken place, but why an organism tends to resemble its progenitor and why it tends to vary are as unknown as before the days of Darwin and Wallace. So the Periodic Law is, after all, a mere statement of fact. But it is a statement which has already exerted vipon chemistry an influence as great as that of gravitation and evolution in their respective fields, and is destined more and more to become the very foundation of chemistry. We may hope and confidently expect that it will eventually lead us to the real constitution of matter.