196 



NATURE 



Sjfune 27, 1889 



(2) The recognition that the relations between the atomic 

 weights of analogous elements were governed by some 

 general law. Many chemists, and more especially Dumas, 

 Gladstone, and Strecker, had drawn attention to the 

 numerical relationship existing between correlated groups 

 of elements, but no one before Newlands in England, and 

 De Chancourtois in France, had sought to generalize this 

 conception, and to extend it to all the elements by con- 

 sidering their properties as functions of their atomic 

 weights. (3) A more accurate knowledge of the relations 

 and analogies of the rarer elements, such, for example, as 

 that given to us by Roscoe in the case of vanadium, and 

 by Marignac in that of niobium. The law of periodicity 

 was the systematized expression of these data ; it was, to 

 use Mendeleeff's language, " the direct outcome of the 

 stock of generalizations of established facts which had 

 accumulated by the end of the decade 1860-70." 



We can only very rapidly allude to some of the more 

 striking services which RIendeleeff's generalization has 

 rendered to science during the twenty years of its 

 existence. By a more systematic arrangement and co- 

 ordination of the known chemical elements, it has not 

 only indicated the existence of new forms of elementary 

 matter, but it has pointed out the probable sources of 

 the undiscovered substances, and has enabled us to know 

 their properties even before we have knowledge of their 

 existence. It was this power of divination inherent in the 

 law which, perhaps more than any other feature, first 

 attracted attention to it, and quickened the interest with 

 which its development was regarded by men of science. 

 There are now three instances of elements of which 

 the existence and properties were foretold by the periodic 

 law : (i) that of gallium, discovered by Boisbaudran, 

 which was found to correspond with the eka- aluminium of 

 Mendeleeff ; (2) that of scandium, corresponding to eka- 

 boron, discovered by Nilson ; and (3) that of germanium, 

 tvhich turns out to be eka-silicium, by Winckler. No one 

 who was present on the occasion of the delivery of the 

 Faraday Lecture will forget the enthusiasm which followed 

 the reading of these words of Mendeleeff' s : " When, in 

 1 87 1, 1 described to the Russian Chemical Society the 

 properties, clearly defined by the periodic law, which such 

 elements ought to possess, I never hoped that I should 

 live to mention their discovery to the Chemical Society of 

 Great Britain as a confirmation of the exactitude and the 

 generality of the periodic law." 



Up to the time of the formulation of the law, the 

 determination of the atomic value or valency of an 

 element was a purely empirical matter, with no apparent 

 necessary relation to the atomic value of other elements. 

 But to-day this value is as much a matter oi a prioriknow- 

 ledge as is the very existence of the element or any one of 

 its properties. Striking examples of the aid which the law 

 affords in deterniining the substituting value of an element 

 are presented by the cases of indium, cerium, yttrimn, 

 beryllium, scandimn, and thoritcm. In certain of these 

 cases, the particular value demanded by the law, and the 

 change in representation of the molecular composition of 

 the compounds of these elements, have been confirmed 

 by all those experimental criteria on which chemists are 

 accustomed to depend. One of the most interesting 

 instances of the kind is seen in the example of uranium, 

 the atomic weight of which was formerly regarded as 



120, then as 180, but which, on the authority of the 

 periodic law, is now established as 240, a value completely 

 confirmed by the independent experiments of Zimmer- 

 mann and Rammelsberg. Uranium has a special interest 

 in being the last term in the series : no element of higher 

 atomic weight is at present known. 



As examples of the value of the law in enabling us to 

 correct the atomic weights of elements whose valencies 

 and true position were well known, we may cite the cases 

 of gold, tellurium, and titanium, the values of which 

 were apparently higher than those demanded by it. In 

 each of these cases a redetermination of the atomic 

 weight has resulted in a value which is in conformity with 

 the previsions of the periodic law. 



The law has, moreover, enabled many of the physical 

 properties of the elements to be referred to the same 

 principle of periodicity. At the Moscow Congress of 

 Russian physicists in August 1869, Mendeleeff pointed 

 out the relations which existed between the density and 

 the atomic weights of the elements ; these were subse- 

 quently more fully examined by Lothar Meyer, and are 

 embodied in the well-known curve in his " Modern Theories 

 of Chemistry." Similar relations have been discovered in 

 certain other properties, such as ductility, fusibility, hard- 

 ness, volatility, crystalline form and thermal expansion ; 

 in the refraction equivalents of the elements, and in their 

 conductivities for heat and electricity ; in their magnetic 

 properties and electro-chemical behaviour ; in the heats 

 of formation of their haloid compounds ; and even in such 

 properties as their elasticity, breaking stress, &c. 



In the Faraday Lecture, Mendeleeff indicated thebearing 

 of the law of periodicity upon the doctrine of constant 

 valency, and especially on the conception of a primordial 

 matter. The mind almost instinctively clings to the 

 notion that the law can only find its rational interpretation 

 in the idea of unity in the formative material, and it is not 

 surprising that the promulgation of the law has been 

 heralded by some as the most convincing proof of the 

 validity of the Pythagorean conception that experiment 

 has yet been able to adduce. But the author of the 

 periodic law will not admit that his generalization has 

 either sprung from this conception or has any relations 

 towards it. " The periodic law, based as it is on the sohd 

 and wholesome ground of experimental research, has been 

 evolved independently of any conception as to the nature 

 of the elements ; it does not in the least originate in the 

 idea of an unique matter; and it has no historical con- 

 nection with that relic of the torments of classical thought ; 

 i and therefore it affords no more indication of the unity of 

 I matter or of the compound nature of the elements than 

 do the laws of Avogadro and Gerhardt, or the law of 

 specific heats, or even the conclusions of spectrum 

 I analysis. None of the advocates of an unique matter has 

 I ever tried to explain the law from the standpoint of ideas 

 I taken from a remote antiquity, when it was found con- 

 venient to admit the existence of many gods— and of an 

 ! unique matter." 



No record of Mendeleeft's intellectual activity would 

 be complete without some reference to his influence on 

 the development of the industrial resources of Russia. 

 In 1863 he brought out the first encyclopaedia of chemical 

 technology of any magnitude which the literature of that 

 country possessed, and he has been frequently com- 



