February 3, 1910] 



NA TURE 



4'5 



dependent upon them. The first step in this direction was 

 taken by J. A. R. Newlands, who, after some preliminary 

 attempts in 1864-5, discovered that when tlie elements are 

 placed in the order of the numerical value of their atomic 

 weights, corrected as advised by Cannizzaro, the eighth 

 element starting from any point on the list exhibits a 

 revival of the characteristics of the first. This undoubtedly 

 represents the first recognition of the principle of perio- 

 dicity in the series of atomic weights, but whetlier dis- 

 couraged by the cool reception of his " law of octaves " 

 by the chemical world or from imperfect apprehension of 

 the importance of this discovery, Newlands failed to follow 

 up the inquiry. It was not long, however, before the 

 matter was taken up by others, and doubtless the improve- 

 ments in the estimation of atomic weights, following on 

 the work of Stas, then only recently published, inspired 

 greater confidence in the approximate accuracy of the 

 numbers adopted as atomic weights, and thus encouraged 

 inquiry into their relations. The subject is, indeed, an 

 attractive one, for it involves considerations which lie at 

 the foundations of all our notions respecting the physical 

 constitution of matter, and accordingly we find papers by 

 many chemists dealing with the question of these numerical 

 relations. Odling especially seems to have given much 

 thought to the sulJject, and, ignoring Newlands's previous 

 attempts, he drew up towards the end of 1864 ' a table 

 containing a list of all the then well-known elements, 

 arranged horizontally in the order of their generally 

 accepted groups, and perpendicularly in the order of their 

 several atomic weights. He concludes an article in Watts 's 

 Dictionary a few months later with these words : — 

 " Doubtless some of the arithmetical relations exemplified 

 in the foregoing table are merely accidental, but, taken 

 altogether, they are too numerous and decided not to 

 depend on some hitherto uinecogiiised law." It is 

 important to note the words I have italicised. 



Such, then, was the state of knowledge about this time. 

 Evidently the way w^as being prepared, but the prophet 

 had not made his appearance — the seer who could look 

 with the eyes of confidence beyond the clouds of uncertainty 

 which obscured all ordinary vision. 



In March, 1869, Mendel^eff communicated to the Russian 

 Chemical Society an enunciation of the principle of perio- 

 dicity 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 modific.i- 

 lions which he immediately afterwards introduced into the 

 arrangement. 



Mendeleeff's First Table of the Elements. 



Ti = 50 2r - 90 ? = 1 80 



V =51 Nb= Q» Ta =182 



Cr = 52 Mo= 96 W =iP6 



Mn = 55 Rh=l04'4 Pt =I97'4 



Fe = 56 Ru=io4-4 Ir =19! 



Ni = Co = 59 Pd=io6-6 Os =199 



Cu ^ 63'4 Ag =108 Hg =200 



r:e = 9-4 Mg =24 



Al 



Ca: 



U =116 Au =197? 



Sn =118 



Sb =122 Bi =210? 



61 •; 



? = 68 



■? = 70 



As = 75 



Pe = 79-4 Te =123? 



Br = 80 I =127 



Rb = 85-4 Cs ^133 Tl =204 



Sr = 87-6 Iia'=i37 Pb =207 



C =12 Si =23 



N =14 P =3, 



O =16 S =3! 



F =19 CI =3V^ 



Li = 7 Na = 23 K =39 



?Er —56 La = 94 



■?Yt =60 I'i =95 



?ln =756 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, Mendel^eff 

 deduced the fundamental principle which he expressed as 

 follows : — The elements arranged according to the magni- 

 tude of atomic weight show a periodic - change of proper- 

 ties. 



1 Quarl. J. Sci., 1864, i, 643 ; and Watts's Dictionar. 



? .^^. ^" error in the German translation does 

 original, inasmuch as the Russian word for periodic 

 weise " (gradual). 



NO. 2I0I, VOL. 82] 



I the 



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. How- 

 ever, Mendelfeff, 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, 18S9). 



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 with 

 properties, but was the first to indicate its character, and, 

 as himself (" Principles," 1905, ii., p. 28) has pointed 

 out, he was the first " to foretell the properties of un- 

 discovered 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 Mendeli5eff'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 

 bv contributing to Liebig's .4Hiia;<;K a paper containing a 

 table, substantially identical with that of Mendeli^eff, 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 generalisations have often resulted from the 

 gradual accumulation of facts which, after remaining for 

 a time isolated or confused, have been found to admit of 

 coordination into a comprehensive scheme, and, this once 

 clearlv 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 dis- 

 coverv of oxygen by Priestley and Scheele and the utilisa- 

 tion of this knowledge by Lavoisier. In each case much 

 preparatory work had been done, and a body of know- 

 ledge had been gradually accumulated w^hich, when duly 

 marshalled 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 as 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 recognised consequences of the system. We 

 mav therefore pass Hghtly 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 Mendel^ff himself (Faraday lecture). We may also 

 pass over the applications of 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 .\ddress to Section B, British 

 .Association, 1897, and Proc. Roy. Soc, 1S9S, Ixiii., 



437)- ' , , , 



It will be more profitable to consider a few of the 

 difificulties which still encumber the application of the law, 

 and which, while limiting our acceptance of it in an un- 

 qualified form as applic.ible 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. .'\n examination of the facts seems, 

 however, to indicate the possibility of some other principle, 



