6o 



NATURE 



[September i8, 1919 



the dements has been the specific heat which, as the 

 discovery of Dulong and Petit showed a hundred 

 years ago, varies in the case of solid elementary 

 bodies inversely with their atomic weights; or, as 

 it is more usually expressed, the solid elements have 

 the same atomic heat. The investigation of the 

 exceptions to this empirical rule brought out the fact 

 that the specific heat is influenced by temperature, 

 and the study of the influence of low temperatures 

 led Sir James Dewar to the discovery that at about 

 50° Absolute the atomic heats of the' elements are a 

 periodic function of the atomic weights. Further, the 

 graphic representation of this relation gives a curve 

 very similar in its course to that of the atomic volume 

 curve. So that the specific heat is another of the 

 physical properties to fit into the periodic scheme. 



The necessity for a revision of the atomic weights 

 of certain elements, as pointed out by Mend<'leeff, has 

 induced several workers to direct their energies to 

 the solution of the problems indicated, so that in our 

 present-day tables many of the anomalies of position 

 and sequence which existed in the earlier schemes 

 have disappeared. Tellurium has still resisted all 

 attempts to bring it into order, with an atomic weight 

 less than that of iodine, which its association with 

 sulphur and selenium demands. The interesting 

 attempts to decompound tellurium have -so far re- 

 mained unfruitful. 



But undoubtedly the most fascinating feature of 

 the periodic system is that "it allows the discovery 

 of many now elements to be foreseen." This and the 

 manner in which Mendel^etT, in full conviction of the 

 truth of the "periodic law," boldly assigned properties 

 to those elements required to fill then blank spaces in 

 the table of the elements, and the verification within 

 twenty years in three instances of these prophetic 

 specifications have contributed to the recognition and 

 firm establishment of the "periodic law" as an 

 article of belief in chemical philosophy, and to make 

 it the mainspring and inspiration of the greater part 

 of modern inorganic research. 



The discovery of argon, the announcement of which 

 formed a notable feature in the proceedings of the 

 association at the Oxford meeting in 1894, and the 

 recognition in it of an element with an atomic weight 

 of 40, raised doubts in the minds of some as to the 

 validity of the scheme of the elements based upon 

 the periodic law. It was indeed a time of testing 

 the faith. The suggestion that argon would prove 

 to be a modified form of nitrogen was brushed aside 

 bv the incontrovertible establishment of it as an 

 element, endowed only with specific physical pro- 

 perties and distinguished from all known elements 

 bv its lack of any of those activities which charac- 

 terise the remaining elements. But argon was not 

 destined to enioy a splendid isolation for long. The 

 researches of Sir W. Ramsay soon brought helium to 

 earth, and he and his colleagues provided a number 

 of companions for argon. So, in a very short period, 

 was recognised the existence of a group of gaseous 

 elements forming a natural family, the molecules of 

 which are monatomic, the members of which are dis- 

 tinguishable by their spectra and atomic weights, but 

 are all in agreement in their unreadiness to take part 

 in anv chemical change. This inertness or nonvalence 

 provided a simple means of reconciliation with the 

 periodic scheme of the elements, as all that was 

 required was simply to add to the eight groups of the 

 table of elements a zero group containing helium, 

 neon, argon, kr>'pton, and xenon, and with niton, 

 the emanation from radium, as a recent addition. If 

 we are to accept Mendel^eff's suggestion, the zero 

 group should contain a member lighter than hydrogen 

 in series i., and in a zero series a still lighter repre- 

 NO. 2603, VOL. 104] 



sentalive of the elements of the zero group, which 

 he has postulated as the "aether" of the physicist. 



Thus the discovery of argon has formed a starting- 

 point in the development and a justification of the 

 natural system of the elements, but it still remains, 

 to make the tabulation complete, that provision should 

 be made for the accommodation of the rare earths. 

 The paper published by \\'erner in 1905, under the 

 title "A Contribution to the Development of the 

 Periodic System," shows how this can be satisfac- 

 torily accomplished. 



The elements of the argon group form a valuable 

 extension to the periodic system, and the knowledge 

 acquired in the investigation of these substances has 

 proved serviceable in the solution of problems in the 

 realms of science and of industry. The knowledge of 

 the properties and behaviour of helium was destined 

 soon to play a part in the solution of the riddle of the 

 radio-active elements, whilst it is specially noteworthy 

 that argon, the "idle one," should have been pressed 

 into industrial service. 



This fact suggests the thought that idleness has its 

 uses, and at the present time how satisfactory would 

 it be were we able to find useful application for a 

 quality which appears to be plentifully and widely 

 distributed in this country. 



The history of helium is still more astonishing, for 

 not until thirty years after its existence had been 

 surmised from spectroscopic observations of the sun 

 was this element found to have a terrestrial existence, 

 and now, as one of the achievements of science during 

 the war, we may look on its production in bulk as a 

 commercial proposition. Moreover, we are told " that 

 the advances made in the production of helium war- 

 rant the opinion that, had the war continued after 

 November 11, iqi8, supplies of helium at the rate of 

 2,000,000 cub. ft. per month would have been pro- 

 duced within the Empire and the United States, and 

 helium-filled aircraft would have been in servic<;" 

 (Nature, July 17, 1919). 



Some of the speculations to which the periodic 

 system of the elements has given rise have been the 

 subjects of communications to this section. 



.\t the Aberdeen meeting Carnelley, whom I have 

 already mentioned as an ardent worker in this field, 

 gave an account of a scheme based on the conception 

 that the elements are composite, having relations 

 similar to those exhibited by the paraffin hydrocarbons 

 and the isologous series of radicals derived from them. 

 He regarded the elements, other than hydrogen, as 

 made up of two simple elements, A and B. A he 

 identified with carbon, with the atomic weight of 12, 

 and B was assumed to have a negative atomic weight 

 of 2. 



In the following year, at Birmingham, Sir W. 

 Crookes devoted his address to this section to an 

 exposition of his ideas of the " genesis of the ele- 

 ments," a subject to which he on many subsequent 

 occasions returned, and amplified in the light of 

 recent discovery. The process of evolution of the 

 elements from a primal "protyle" is depicted as 

 taking place in cycle after cycle, in each cycle the 

 " unknown formative cause " scattering along its 

 journey clusters of particles corresponding with the 

 atoms of the "elements," forming in this way a 

 series such as that beginning with hydrogen and 

 ending with chlorine ; a repetition of the movement 

 under somewhat altered conditions giving rise to a 

 series of similarly related elements, and thus homo- 

 logy, which is shown by the members of the natural 

 families, is provided for. 



The investigations of Sir J. J. Thomson on the 

 discharge of electricity through gases have established 

 the divisibility of the atoms, and in his "Corpuscular 



