IN March, 1869, Mendeleeff communicated to the Russian Chemical Society an enunciation of the principle of periodicity and a statement of some of the consequences of this recognition of. the relation of properties to atomic weight throughout the whole range of the known elements, and this statement was accompanied by a table which, while it bears a close resemblance to Odling's table of 1864, was apparently connected in his mind with an idea which became clearer and more decisive in the modifications which he immediately afterwards introduced into the arrangement. Mendeleeff's First Table of the Elements. Ti - 50 Zr = 90 ? = 1 80 V = 51 Nb = 94 Ta = 182 (!r =52 Mo = % W = 186 Mn = 55 Eh = 104.4 Pt = 197.4 1<Y = 56 Hu = 104.4 Ir = 198 Ni = Co = 59 Pd = 106.6 Os = 199 H =1 C'n = 63.4 Ag=108 Hg=200 He = 9.4 Mg = 24 Zn = 05.3 Oil = 113 B =11 Al =27.4 » = 68 IT =116 All =197? C = 12 Hi = 28 » = 70 Sn = 118 N =4 1' =81 A3 - 75 Sb = 122 Bi = 210 ? O =16 S =32 Se = 79.4 To = 128 ? F = 19 01 = 35.5 Br - 80 1 = 127 1,1 =7 Na =23 K =39 Rb =- 85.4 Cs 133 Tl =204 (!a =40 Si = 87.6 Ba = 137 Pb = 207 i = 45 Ce = 92 VEr = 56 La = 94 m = 60 Di = 95 fin = 75.6 Th = 118 From this arrangement, in which the elements are placed in vertical columns, according to increasing atomic weight, so that the horizontal lines contain analogous elements again according to increasing atomic weight, Mendeleeff deduced the fundamental principle which he expressed as follows: The elements arranged according to the magnitude of atomic weight show a periodic! change of properties. Previous students of the subject had been, for the most part, struck with the relations obviously subsisting between the members of the several natural families of elements, but had, with few exceptions, failed to perceive that there must be a general law binding the whole together. However, Mendeleeff, with that noble sentiment of justice which always animates the truly scientific mind, admits that the idea of a general law had already been foreshadowed by others (Faraday lecture, 1889). Mendeleeff's table of 1869 was subsequently in 1871 modified so as to assume the form with which we have all been so long familiar, and which is to be found in every modern text-book. Thus it may be claimed for Mendeleeff that he was actually the first, not only to formulate a general law connecting atomic weights * The Mendeleeff Memorial Lecture delivered before the Chemical Society on October 21, 1909, by Sir William A. Tilden, F.R.8. Abridged from the Journal of the Society for December, 1909. t Here an error in the German, translation does an Injustice to the original, inasmuch as the Russian word for periodic is rendered “stufenweise” (gradual). with properties, but was the first to indicate its character, and, as he himself (“Principles,” 1905, ii. p. 28) has pointed out, he was the first “to foretell the properties of undiscovered elements, or to alter the accepted atomic weights” in confidence of its validity. The time was, in fact, ripe for the enunciation of this general principle, and, the suggestion once given, the relations embodied in the law could not fail to attract other chemists. Accordingly, in December. 1869, Lothar Meyer, with such knowledge of Mendeleeff's scheme as could be derived from the imperfect German version of his paper of the previous March, proved himself a convinced exponent of the idea by contributing to Liebig's Annalen a paper containing a table, substantially identical with that of Mendeleeff, and his famous diagram of atomic volumes, which, more clearly even than the tabular scheme, illustrates the principle of periodicity. The history of science shows many instances of the same kind. Great generalizations have often resulted from the gradual accumulation of facts which, after remaining for a time isolated or confused, have been found to admit of co-ordination into a comprehensive scheme, and, this once clearly formulated, many workers are found ready to assist in its development. The case is nearly parallel to the recognition of the operation of natural selection by Darwin and Wallace, or it might be compared to the discovery of oxygen by Priestley and Scheele and the utilization of this knowledge by Lavoisier. In each case much preparatory work has been done, and a body of knowledge had been gradually accumulated which, when duly marshaled and surveyed by the eye of a master, could scarcely fail to reveal to him the underlying principle. The full consequences, however, would appear only to a few. I regard it unnecessary, in the presence of the fellows of the Chemical Society, to review with any detail the multitudinous applications of the scheme of the elements constructed on the basis of the periodic law. These are the commonplaces of modern theoretical chemistry. They are embodied in every text-book of any importance, and are related by every lecturer and teacher as familiar and indisputably recognized consequences of the system. We may, therefore, pass lightly over the story of the prediction by Mendeleeff of the properties of undiscovered elements, confirmed so remarkably by the discovery of scandium, gallium, and germanium, and related in dramatic language by Mendeleeff himself (Faraday lecture). We may also pass over the applications ot the system to the correction of atomic weights, illustrated by the case of beryllium, the recognition of previously unnoticed relations, and the discovery of new elements, notably the companions of argon (Ramsay, Presidential Address to Section B, British Association, 1897, and Proc. Roy. Soc, 1898, Ixiii., 437). It will be more profitable to consider a few of the difficulties which still encumber the application of the law, and which, while limiting our acceptance of it in an unqualified form as applicable to the whole of the elements, tempt the speculative mind to wander in wide fields of conjecture. Can it be truly said that the elements arranged in the order of their atomic weights show without exception periodic changes of properties? This question has been propounded already, but has never been fully discussed, even by Mendeleeff. An examination of the facts seems, however, to indicate the possibility of some other principle, which, while it does not supersede the periodic scheme, would, if it could be recognized, supplement it. From a consideration of the almost unbroken sequence in the atomic weights of the known elements, it seems probable that few additional elements are to be expected, except possibly on following Mo and another following W, save in the region from Bi to Ra. This suggests the remark that, after all, it is not necessary to assume that the materials of which the earth consists should necessarily include a sample of every possible element indicated by such a scheme. Some which are missing from terrestrial matters may perhaps be responsible for phenomena recognizable by the spectroscope in stars or nebulae far distant in cosmical space. The unexpected, however, often happens, and, remembering the discovery of terrestrial helium, it is permissible to hope that some of the vacant spaces may hereafter be filled by earthly occupants. There is one important point to be noted here, namely, that if the so-called rare earth metals, praseodymium, neodymium, samarium, gadolinium, terbium, dysprosium, erbium, ytterbium, and others of which the existence is doubtful, do lie in the position indicated, the original statement of the periodic law breaks down at this point. One result of the recognition of the periodic law is-that theories concerning the genesi. of the elements have received a stimulus previously unknown. It is, however, interesting to note the attitude of Mendeleeff toward this question, and the small extent to which this attitude appears to have become modified with the lapse of time. When, in 1889. twenty years after the discovery of the law, he composed the Faraday lecture, he seems to have regarded speculation in this direction as a kind of abuse of the periodic system. Fifteen years later, after the discovery of the argon group of elements, of the phenomena of radio activity, and of radium, it became necessary to consider the relations of these substances to the periodic scheme. In a remarkable article contributed to the new Russian Encyclopaedia, and subsequently printed as Appendix iii. to the “Principles” (English edition, 1905), Mendeleeff gives a new table of the elements, in which places are found, not only for the argon group and radium, but for two hypothetical elements, which are placed before helium and designated x and y. The y in the table is supposed to be an analogue of helium, and may be identified hereafter with “coron-ium,” which has been recognized in the sun's coronal atmosphere. This gas would have, according to Mendeleeff, a density about 0.2, and, therefore, a molecular weight about 0.4, or about one-tenth that of helium. x is the “ether” of the physicist, for which Mendeleeff, disregarding conventional views, supposes a molecular structure. He also assumes that, like the argon group, this element is chemically inert and possesses a very low density and atomic weight, estimated at 0.000,000,000,053. Chemists and physicists have, however, found it impossible to resist the fascination of this problem, and accordingly there have been many hypotheses as to the original of the elements and the nature of their connection with one another. These seem to be inseparable from the periodic scheme itself, which at once provokes the inquiry: Why do these numerical relations occur, and what is the meaning of them if they do not point to a common genesis or the operation of some process of evolution? Hypotheses concerning the evolution of the elements have hitherto been usually based on the assumption that the successive stages of condensation of elemental matter proceed from a single primary stuff, which by a process analogous to polymerization among carbon compounds gave rise to atoms of greater and greater mass, which were stable at the prevailing and any low temperature. The physical cause of the successive condensations is supposed to be a falling temperature. It is, of course, possible to imagine that if to the stuff of which hydrogen atoms consist are added successive portions of matter of the same kind, stable structures may at intervals result which we know as the atoms of the elements helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, and fluorine, provided the idea of internal structure in these atoms is allowed. Otherwise, from the mere accretion of matter upon a central nucleus, there seems no sufficient reason why there should not have been formed an indefinite number of intermediate masses corresponding to an indefinite number of what would be called elements. Further, it is difficult to understand why simple increase of mass should change, say, oxygen into fluorine, while a further addition of the same kind should change negative flourine into inert neon or positive sodium. The possibility of the condensation of a single “protyl” so as to produce, at successive though unequal stages of cooling, the elements known to the chemist, has been most ably discussed long ago by Sir William Crookes. This hypothesis, however, was put forward long before the work of Sir J. J. Thomson and his school was given to the world and the electron was accepted as a physical reality. The hypothesis that one elemental stuff may give rise to the whole array of known elements by a process of condensation accompanied by a loss or gain of electrons, the mass of which is approximately one-thousandth of the mass of an atom of hydrogen, forms the subject of a paper by Mr. A. C. G. Egerton in a recent number of our Transactions (1909, xcv., 239). The atomic weights calculated by his formula agree closely with the experimental atomic weights of the first fifteen elements, but the hypothesis gives no explanation of the facts observed in the physical properties of the elements arranged according to the Mendeleeff scheme, their alternation of odd and even valency, the transition from positive on one side of the table to negative on the other, the periodicity of properties shown by the sudden change of character in passing from fluorine to the next element, whether it be neon or sodium. Another paper by Messrs. A. C. and A. E. Jessup (Phil. Mag., 1908 [vi.], xv., 21) has recently provided a hypothesis of an entirely different character. From a study of the spectra of the nebulas, these authors have been led to assume the existence of two hitherto unrecognized elements, to which the names protoglu-cinum and protoboron are assigned. These with hydrogen and helium are supposed to represent four initial substances, or protons, which, by condensation directly or indirectly, give rise to all the rest of the elements. The arguments of these authors are ingenious, but rather artificial in view of the fact that the number of groups in the periodic scheme to be provided for is greater than four. In the Mendelgeff chart of the elements there is nothing more striking than the gathering of the negative elements toward what may be called the N.E., and the segregation of the positive elements toward the S.W., the center of the intermediate territory being occupied by elements which play a more or less undecided part. I have elsewhere (Presidential Address, 1905, Trans., lxxxvii., 564) directed attention to the fact that carbon, at any rate, is not directly deposited by electrolysis' from any of its compounds, with positive hydrogen on the one hand, or negative chlorine on the other. I be- lieve the same is true of silicon, these two elements standing in a middle position between the extremes occupied by lithium and fluorine respectively. If we assume that atoms are made up of two parts (protyls), positive and negative, in proportions which determine by the preponderance of one or the other whether the element shall exhibit the positive character of a metal like lithium or the negative character of a halogen, we arrive at a hypothesis which recalls the ideas put forward nearly a century ago by Berzel-ius. His views are familiar to every student of the history of chemistry, but have long been relegated to the lumber-room of worn-out doctrine. The last few years have, however, given us the remarkable experimental investigations of J. J. Thomson already referred to, and the new conceptions concerning the nature of atoms, which revive the fundamental idea that they are made up of two components. * Setting out the known elements in the order of the numerical value of their atomic weights, we find that between the first three elements, H = 1, He = 4, and Li = 7, the difference, 3, is greater than would be expected by comparison with the difference noticed between the elements of greater atomic weight which immediately follow them. In order to satisfy the hypothesis just put forward, there appears to be wanting an element which should stand in the same relation to fluorine as hydrogen to lithium. This would have an atomic weight 2-7 approximately. Whether this exists, and whether its existence is indicated by the unappropriated spectral lines of nebulas or corona, can only be a matter of conjecture. Mendeleeff, in his (1905) latest speculations concerning the possibility of still undiscovered elements, has suggested the existence of a new element of the halogen group with an atomic weight about ;t but, as already sufficiently shown, he accepted no hypothesis which involved any idea of the composite nature of the elements. It would, therefore, have been foreign to his system to employ this element in any such manner. The conceptions presented to us in J. J. Thomson's work permit of several supplementary hypotheses, especially the idea that if atoms are really made up of smaller corpuscles these are not thrown together in confusion, but, as he has shown, must be distributed within the mass in a definite order, which is determined by the attraction of the electro-positive shell and the self-repulsion of the negative corpuscles included in it. Once the idea of structure within the atom is admitted, the possibility presents itself of there being for the same mass more than one arrangement corresponding to what is called isomerism in compounds. I have dwelt at some length on these various hypotheses, because the discussion of the subject to which they relate indicates, in my opinion, one of the consequences of the promulgation and general acceptance of the periodic scheme of the elements. This is, however, not the only result of the recognition of its validity and usefulness by chemists generally. That the elements stand in a definite relation to one another implies that their compounds also fall into their places in an orderly system, and consequently a basis is provided for the complete systematization of the whole science of chemistry. There is scarcely a treatise on chemistry which does not bear evident witness to this influence; and this is perhaps not the least among the services rendered by this generalization, for not only is the learner enabled to remember a much larger number of facts than previously, but he is led to perceive a connection between phenomena and processes which was almost entirely wanting so long as practical chemistry consisted mainly of a bundle of recipes. Here it is fitting that we should glance at the famous treatise by Mendeleeff himself, The Principles of Chemistry,” of which we possess three editions in English, the last of which, issued in 1905, is a rendering of the seventh edition (1903) of the original. An eighth Russian edition began to be issued in 1905, but is incomplete. To this remarkable book it is impossible to do justice in a brief notice or to communicate to those who have not read it an adequate impression. Clearly it is a work of genius, but such works are not always the most suitable for beginners, though . for the advanced student nothing can be more inspiring. The “Principles” embody in reality two distinct treatises, for the text, which is written in an easy style, open to quite straightforward reading, is accompanied by notes * Carnclley, In 1885 (Brit. Assoc. Reports), brought forward the idpa “that the elements are not elements in the strict sense of the term, but are, in fact, compound radicals made up of at least two simple elements, A and B.” The element A was supposed to be identical with carbon, while to B was assigned a negative weight, —2, and it was suggested that it might be the ether of space. C. S. Palmer (Prof. Colorado Scient. Soc.) assumed the existence of two sub-elements, to which he gave the names “kalidium” and “oxidium,” and his views appear to have a general resemblance to the hypothesis suggested in the text. The original article is abstracted in Venable's “Periodic Law,” and is referred to in footnotes in Palmer's translation of Nernst's “Theoretical Chemisti7 “ t It may also, perhaps, be worthy of note that Mr. Eager-ton's calculations (lee. cit.) lead him to postulate an element of nearly this atomic weight, namely, 2.9844, although his paper gives no indication as to its character. which are often more voluminous and usurp entire p.ages. Even the preface is attended by these commentaries, which are all interesting as showing the spirit of the writer and the restless activity of his mind. Little more remains to be said. In the seventeenth century Robert Boyle taught us how to distinguish elements from compounds, and how to give the word ''element” a definite connotation clearly distinguishing it from the elusive and fantastic language of the alchemists. In the eighteenth century Lavoisier showed the true nature of the most familiar of chemical compounds, namely, acids, bases, and salts, and helped to lay the foundation of quantitative chemistry. At the beginning of the nineteenth century Dalton gave to chemistry the atomic theory, of which it is not too much to say that it provided the scaffold by the aid of which the entire fabric of modern theoretical chemistry has been built up. Sixty years later this conception, developed and adorned by the labors of an army of earnest workers, has been shown to us in a brilliant new light thrown over the whole theory by Mendeleeff. The views of Boyle, of Lavoisier, and of Dalton have been corrected by experience and broadened by extended knowledge, but the fundamental and essential parts of their ideas remain, and their names are im mortal. In like manner the expression of the periodic law of the elements as known to the present generation is destined, we may believe, to be absorbed into a more compresensive scheme by which obscurities and anoma-ies will be cleared away, the true relations of all the elements to one another revealed, and doubts as to the doctrine of evolution resolved in one sense or the other; but as with the atomic theory itself, there is no reason to doubt that the essential features of the periodic scheme will be clearly distinguished through all time, and in association with it the name of Mendeleeff will be for ever preserved among the fathers or founders of chemistry. In an article published in the Z. physik. Chem., A. Coehn and H. Becker describe some investigations of the photochemistry of sulphuric acid. The authors studied the formation of sulphur trioxide from sulphur dioxide and oxygen under the influence of the radiation from a quartz mercury lamp. Experiments made with a quartz reaction vessel mounted within a mercury lamp showed that the formation of sulphur trioxide proceeds fairly rapidly even at the ordinary temperature and more rapidly at higher temperatures. The equilibrium is quite distinct from that attained in daylight. With the mixture, 2SO;, : 0 2 , equilibrium is attained with a production of 65 per cent of sulphur trioxide, and this equilibrium can also be attained starting from sulphur trioxide. At 160 deg. C, with the gases confined in the reaction chamber, equilibrium was attained in 1 hour. In daylight at temperatures below 450 deg. C. the equilibrium condition corresponds with practically 100 per cent of sulphur trioxide. Again the light-equilibrium (attained on exposure to the radiation from the mercury damp) is not displaced by a rise of temperature up to 800 deg. C, whereas the temperature of dark-equilibrium (in daylight) is displaced considerably by a rise of temperature above 450 deg. C. With a mixture in the proportions, SO;, : 0 3 = 1 : 13, for example, at 800 deg. C, the yield of sulphur trioxide is 80 per cent on exposure to the rays from the mercury lamp, whereas in daylight, the yield is 44 per cent of sulphur trioxide. The temperature coefficient of the velocity of the photochemical reaction was found to be 1.2. The authors consider that a technical photochemical process for the production of sulphuric acid is quite feasible, since on using air in place of oxygen, no oxides of nitrogen are formed. Some experiments were made with a special mercury-lamp composed essentially of a quartz tube, 115 centimeters long and 1.8 centimeter diameter. This was inclosed in a tube of opaque, English quartz, 100 centimeters long and 5 centimeters diameter, the annular space between the two serving as a reaction chamber. At 450 deg. C. and with the gaseous mixture passing at the rate of 100 to 150 cubic centimeters per minute, yields of 67.4, 70.8, 92, and 90 per cent respectively of sulphur trioxide were obtained, with 0.78, 0.66, 8.7, and 9.3 molecules of oxygen per molecule of sulphur dioxide. The State railway from Magdeburg to Halle, via Zerbst and Leipzig, states the Electrical Engineer, will soon be electrified throughout its entire length, and also the branch line from Dessau to Bilterfeld, which is about 16 miles long. The total length of the line between Magdeburg and Halle is about 96 miles. It taps all the important brown coal fields in the neighborhood of Mulde, which urgently require better and cheaper means of transport. The power will be provided by a station at the small Prussian village of Muldenstein. which has been selected because brown coal for fuel is abundant there. The current used for the electric locomotives will be a high-tension alternating current, supplied at. 10,000 volts. The whole cost will exceed one million sterling, at all events, but it is said that it may be considerably more.