which govern the formation and properties of the oxides and of all the compounds of the elements, but also a fresh and exact means for recognising the analogy of elements. Analogous elements give compounds of analogous types, both higher and lower. If CO, and SO, are two gases which closely resemble each other both in their physical and chemical properties, the reason of this must not be looked for in an analogy of sulphur and carbon; but it lies in that identity of the type of combination, RX,, in which both oxides appear, and in that influence which a large mass of oxygen always exerts on the properties of its compounds. In fact, there is little resemblance between carbon and sulphur, as is seen not only from the fact that CO, is the higher form of oxidation, whilst SO, is able to further oxidise into SO3, but also from the fact that all the other compounds—for example, SH, and CH,, SCI, and CCI, &c.-are entirely unlike both in type and in chemical properties. This absence of analogy in carbon and sulphur is clearly expressed in the fact that the higher oxides are of different composition, CO, for carbon, and SO, for sulphur. The halogens, which are analogous, give both like higher and lower compounds. So also do the metals of the alkaline earths and of the alkalis. Many such groups of analogous elements have long been known. Thus there are analogues of oxygen, nitrogen, and carbon, and we shall meet with many such groups. But an acquaintance with them involuntarily leads to the questions, what is the cause of analogy and what is the relation of one group to another? If these questions remain unanswered, it is easy to fall into error in the formation of the groups because the notions of the degree of analogy will always be relative, and will not present any accuracy or distinctness. Thus lithium is analogous in some respects to potassium and in others to magnesium; or beryllium is analogous to both aluminium and magnesium. In thallium, as we shall afterwards see and as was observed on its discovery, there is much in kindred with lead and mercury, but some of its properties appertain to lithium and potassium. Naturally, where it is impossible to make measurements one is involuntarily obliged to limit oneself to approximation or comparison, founded on Thus, for instance, the conception of the ortho-acids and of the normal acids will be considered in speaking of phosphoric and phosphorous acids.

As in the further exposition of the periodic law only those oxides which give salts will be considered, I think it will not be superfluous to here mention the following facts relative to the peroxides. Of the peroxides corresponding with hydrogen peroxide, the following are at present known: H2O2, Na2O2, SO, (as HSO,?), K ̧O1, K ̧О1⁄2, CаО, TiO3, Cr2O, CuO2(?), Rb2O2, SrO2, Ag2O2, CsO2, Cs,O, BaO, and MO. It is probable that the number of peroxides will increase with further investigation. A periodicity is seen in those now known because the elements (excepting Li) of the first group, which gives RO, form peroxides, and then the elements of the fourth group seem also to be particularly inclined to form peroxides.

apparent signs which are not distinct and are void of exactitude. But in the elements there is one accurately measurable property, which is subject to no doubt-namely, that property which is expressed in their atomic weights. Its magnitude indicates the relative mass of the atom, or, if we avoid the conception of the atom, its magnitude shows the relation between the masses forming the chemical and independent individuals or elements. And according to the sense of all our physicochemical data, the mass of a substance is that property on which all the remaining properties of matter must be dependent, because they are all determined by similar conditions or by those forces which act in the weight of a substance, and this is directly proportional to the mass of a substance. Therefore, it is most natural to seek for a dependence between the properties and analogies of the elements on the one hand and their atomic weights on the other.

This is the fundamental idea which leads to arranging all the elements according to their atomic weights. A repetition of properties is then immediately observed in the periods of the elements. already familiar with examples of this

We are

The halogens

The substance of the matter is seen in these groups. have smaller atomic weights than the alkali metals, and the latter than the metals of the alkaline earths. Therefore, if all the elements be arranged in the order of their atomic weights a periodic repetition of properties is obtained. This is expressed by the law of periodicity; the properties of the elements, as well as the forms and properties of their compounds, are in periodic dependence or, expressing ourselves algebraically, form a periodic function of the atomic weights of the elements. Table I. of the periodic system of the elements, which

8 The periodic law and the periodic system of the elements appeared in the same form as here given in the first edition of this work, begun in 1868 and finished in 1871. In laying out the accumulated information respecting the elements, I had occasion to reflect on their metrical relations. At the beginning of 1869 I distributed among many chemists a tract entitled, 'An Experimental System of the Elements, based on their Atomic Weights and Chemical Analogies,' and at the March meeting of the Russian Chemical Society, 1869, I communicated a paper On the Correlation of the Properties and Atomic Weights of the Elements.' The substance of this paper is embraced in the following conclusions: (1) The elements, if arranged according to their atomic weights, exhibit an evident periodicity of properties. (2) Elements which are similar as regards their chemical properties have atomic weights which are either of nearly the same value (platinum, iridium, osmium) or which increase regularly (e.g. potassium, rubidium, cæsium). (3) The arrangement of the elements or of groups of elements in the order of their atomic weights,

is placed at the very beginning of this book, is composed according to this law. It is arranged in conformity with the eight types of oxides described in the preceding pages, and those elements which give the oxides RO, and consequently salts RX, form the 1st group; the elements giving R2O, or RO as their highest grade of oxidation enter into the 2nd group, those giving RO, as their higher oxides form the

3

corresponds with their so-called valencies. (4) The elements, which are the most widely diffused in nature, have small atomic weights, and all the elements of small atomic weight are characterised by their sharply-defined properties. They are therefore typical elements. (5) The magnitude of the atomic weight determines the character of an element. (6) The discovery of many yet unknown elements may be expected. For instance, elements analogous to aluminium and silicon, whose atomic weights would be between 65 and 75. (7) The atomic weight of an element may sometimes be amended by aid of a knowledge of those of the contiguous elements. Thus the combining weight of tellurium must lie between 123 and 126, and cannot be 128. (8) Certain characteristic properties of the elements can be foretold from their atomic weights.

The entire periodic law is included in these lines. In the series of subsequent papers (1870-72, for example, in the Transactions of the Russian Chemical Society, of the Moscow meeting of Naturalists of the Petroffsky Academy, and Liebig's Annalen) on the same subject, we only find applications of the same principles, which were afterwards confirmed by the labours of Roscoe, Carnelley, Thorpe, and others in England, of Rammelsberg (respecting cerium and uranium), L. Meyer (respecting the specific volumes of the elements), Zimmermann (respecting uranium), and more especially of C. Winkler (who discovered germanium, and showed its identity with ekasilicon), and others in Germany; of Lecoq de Boisbaudran in France (the discovery of gallium=ekaaluminium), of Cleve respecting the atomic weights of the cerium metals), Nillson (discoverer of scandium=ekaboron), and Nillson and Pettersson (determination of the vapour density of beryllium chloride) in Sweden, and of Brauner (who investigated cerium, and determined the combining weight of tellurium =125), in Austria.

I consider it necessary to state, that in arranging the periodic system of the elements I made use of the previous researches of Dumas, Gladstone, Pettenkofer, Kremers, and Lenssen on the atomic weights of related elements, but I was not acquainted with the works preceding mine of De Chancourtois (vis tellurique, or the spiral of the elements according to their properties and equivalents) in France, and of J. Newlands (Law of Octaves-for instance, H, F, Cl, Co, Br, Pd, I, Pt form the first octave, and O, S, Fe, Se, Rh, Te, Au, Th the last) in England, although certain germs of the periodic law are to be seen in these works. With regard to the work of Prof. Lothar Meyer respecting the periodic law (Notes 12 and 13), it is evident, judging from the method of investigation, and of his statement (Liebig's Annalen Supt. Band 7, 1870, 354), at the very commencement of which he cites my paper of 1869 above mentioned, that he took the periodic law in the same form as it was given by me.

In conclusion to this historical statement I consider it well to observe that no law of nature, however general, has been established at once; its establishment is always preceded by many presentiments, but the acknowledgment of a law does not take place when it is recognised in all its significance, but only when it has been confirmed by experiment, which the scientific man must look to as the only proof of the correctness of his conjectures and opinions. I therefore, for my part, consider Roscoe, De Boisbaudran, Nillson, Winkler, Brauner, Carnelley, Thorpe, and others who verified the adaptability of the periodic law to chemical reality, as the true founders of the periodic law, the further development of which still awaits fresh workers. The efforts made in this direction, which are mentioned in the following notes, cannot for the present be considered as in any way elucidating many questions which involuntarily arise on an acquaintance with this law.

VOL. II.

C

3rd group, and so on, whilst the elements of all the groups which are nearest in their atomic weights are arranged in series from 1 to 12. The even and uneven series of the same groups present the same forms, but differ in their properties, and therefore two contiguous series, one even and the other uneven-for instance, the 4th and 5th-form a period. Hence the elements of the 4th, 6th, 8th, 10th, and 12th, or of the 3rd, 5th, 7th, 9th, and 11th series form analogues, like the halogens, alkali metals, &c. The conjunction of two series, one even and its contiguous uneven series, thus forms one large period. These periods, beginning with the alkali metals, end with the halogens. The elements of the two first series have the least atomic weights, and in consequence of this very circumstance, although they bear the general properties of a group, still they show many peculiar and independent properties. Thus fluorine, as we know, differs in many points from the other halogens, and lithium from the other alkali metals, and so on. These lightest elements may be termed typical elements. They

In the annexed table all the remaining elements are arranged, not in groups and series, but according to periods. In order to penetrate into the essence of the matter, it must be remembered that here the atomic weight gradually increases along a given line; for instance, in the line commencing with K=39 and ending with Br (the 17th)=80, the intermediate elements have intermediate atomic weights.

Even Series.

Mg Al Si P S CI

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br

The same degree of analogy as we know to exist between potassium; rubidium, and cæsium; or chlorine, bromine, and iodine; or calcium, strontium, and barium also exists between the elements of the other vertical columns. Thus, for example, zinc, cadmium, and mercury,

9 This resembles the fact, well known to those having an acquaintance with organic chemistry, that in a series of homologues (Chap. VIII.) the first members, in which there is the least carbon, although showing the general properties of the homo-logous series, still present certain distinct peculiarities.

which are described in the following chapter, present a very close analogy with magnesium. All our further descriptions of the elements will be arranged according to this periodic system. The distribution of the elements in groups, series, and periods is shown in th tables given in this chapter, at the end of the preface, and at the end of the book. For a true comprehension of the matter, it is very important to see at once, that all the given aspects 10 of the distribution

10 Besides arranging the elements (a) in a successive order according to their atomic weights, with indication of their analogies by showing some or other properties (both chemical-for instance, their power of giving one or another form of combination-and physical), both of the elements and of their compounds (as is done in Table III. at the beginning of this volume), (b) according to periods (as is done in Table I. at the commencement of volume I. after the preface), and (c) according to groups and series or small periods (as is done in Table II. and in Table IV. at the end of this volume), I am acquainted with the following methods of expressing the periodic relations of the elements: (1) By a curve drawn through points obtained in the following manner: The elements are arranged along the horizontal axis of abscissæ at distance; from zero proportional to their atomic weights and the values for each of the elements. of some property-for example, specific volumes or melting points are expressed by the ordinates. This method, although graphic, has the theoretical disadvantage that it does not in any way indicate the existence of a limited and definite number of elements. in each period. There is nothing, for instance, in this method of expressing the law of periodicity to show that between magnesium and aluminium there can be no other element with an atomic weight of say 25, atomic volume 13, and in general having properties intermediate between those of these two elements. The actual periodic law does not correspond with a continuous change of properties, with a continuous variation of atomic weight-in a word, it does not express an uninterrupted function-and as the law is purely chemical, starting from the conceptions of atoms and molecules which combine in multiple proportions, with intervals (not continuously), it, above all, depends on there being but few types of compounds, which are arithmetically simple, repeat themselves, and offer no uninterrupted transitions, and therefore each period can only contain a definite number of members. For this reason there can be no other elements between magnesium, which gives the chloride MgCl2, and aluminium, which forms ALX5; there is a break in the continuity, according to the law of multiple proportions. The periodic law ought not, therefore, to be expressed by geometrical figures in which continuity is always understood. Owing to these considerations I have never, and will never, express the periodic relations of the elements by any geometrical figures. (2) By a plane spiral. Radii are traced from a centre, proportional to the atomic weights; analogous elements: lie along one radius, and the points of intersection are arranged in a spiral. This method, adopted by De Chancourtois, Baumgauer, E. Huth, and others, has many of the imperfections of the preceding, although it removes the indefiniteness as to the number of elements in a period. In it one ought only to see a simple endeavour to bring the complex relations under a simple graphic representation, because the number of radii and the formulation of the spiral is not dependent on any conditions. (3) By the lines of atomicity, either parallel, as in Reynolds's and the Rev. S. Haughton's method, or as in Crookes's method, inclined to the right and left of an axis, along which the magnitudes of the atomic weights are counted, the points of the elements are marked off, on the one side the members of the even series (paramagnetic, like oxygen, potassium, iron), and cn the other side the members of the uneven series (diamagnetic, like sulphur, chlorine, zinc, and mercury). On joining up these points a periodic curve is obtained, likened by Crookes to the oscillations of a pendulum, and, according to Haughton, representing a cubical curve. This method would be very graphic did it not require, for instance, that sulphur should be considered as bivalent and manganese as univalent, although