August 31, 1911] 



NATURE 



285 



iodine by Courtois in 1S12, and of bromine by Balard in 

 1826, led to the inevitable conclusion that fluorine, if 

 isolated, should resemble the other halogens in properties, 

 and much later, in the able hands of Moissan, this was 

 shown to be true. 



The modern conception of the elements was much 

 strengthened by Dalton's revival of the Greek hypothesis 

 of the atomic constitution of matter, and the assigning to 

 each atom a definite weight. This momentous step for 

 the progress of chemistry was taken in 1803 ; the first 

 account of the theory was given to the public, with 

 Dalton's consent, in the third edition of Thomas Thom- 

 son's " System of Chemistry " in 1807; it was subsequently 

 elaborated in the first volume of Dalton's own "System 

 of Chemical Philosophy," published in 1808. The notion 

 that compounds consisted of aggregations of atoms of 

 elements united in definite or multiple proportions, 

 familiarisetl the world with the conception of elements as 

 the bricks of which the Universe is built. Vet the more 

 daring spirits of that day were not without hope that the 

 elements themselves might prove decomposable. Davy, 

 indeed, went so far as to write in 181 1 : " It is the duty 

 of the chemist to be bold in pursuit ; he must recollect how 

 contrary knowledge is to what appears to be experience. 

 ... To inquire whether the elements be capable of being 

 composed and decomposed is a grand object of true philo- 

 sophy." And Faraday, his great pupil and successor, at 

 a later date, 1815, was not behind Davy in his aspirations 

 when he wrote : " To decompose the metals, to re-form 

 them, and to realise the once absurd notion of transforma- 

 tion — these are the problems now given to the chemist for 

 solution." 



Indeed, the ancient idea of the unitary nature of matter 

 was in those days held to be highly probable. For attempts 

 were soon made to demonstrate that the atomic weights 

 were themselves multiples of that of one of the elements. 

 At first the suggestion was that oxygen was the common 

 basis ; and later, when this supposition turned out to be 

 untenable, the claims of hydrogen were brought forward 

 by Prout. The hypothesis was revived in 1S42, when 

 Liebig and Redtenbacher, and subsequently Dumas, carried 

 out a revision of the atomic weights of some of the 

 commoner elements, and showed that Berzelius was in 

 error in attributing to carbon the atomic weight 1225 

 instead of 1200. Of recent years a great advance in the 

 accuracy of the determinations of atomic weights ha? been 

 made, chiefly owing to the work of Richards and his pupils, 

 of Gray, and of Guye and his collaborators, and every year 

 an international committee publishes a table in which the 

 most probable numbers are given on the basis of the 

 atomic weight of oxygen being taken as sixteen. In the 

 table for 191 1, of eighty-one elements, no fewer than 

 forty-three have recorded atomic weights within one-tenth 

 of a unit above or below an integral number. My mathe- 

 matical colleague, Karl Pearson, assures me that the 

 probability against such a condition being fortuitous is 

 20,000 millions to one. 



The relation between the elements has, however, been 

 approached from another point of view. After preliminary 

 suggestions by Dbbereiner, Dumas, and others, John New- 

 lands in 1S62 and the following years arranged the 

 elements in the numerical order of their atomic weights, 

 and published in The Chemical News of 1S63 what he 

 termed his law of octaves — that every eighth element, like 

 the octave of a musical note, is in some measure a repeti- 

 tion of its forerunner. Thus, just as C on the third space 

 is the octave of C below the line, so potassium, in 1863 

 the eighth known element numerically above sodium, re- 

 peats the characters of sodium, not only in its physical 

 properties — colour, softness, ductility, malleability, &c. — 

 but also in the properties of its compounds, which, indeed, 

 resemble each other very closely. The same fundamental 

 notion was reproduced at a later date, and independently, 

 by Lothar Meyer and Dmitri Mendeleeff : and to 

 accentuate the recurrence of such similar elements in 

 periods, the expression " the periodic system of arranging 

 the elements " was applied to Newlands' arrangement in 

 octaves. As everyone knows, by help of this arrangement 

 Mendeleeff predicted the existence of then unknown 

 elements, under the names of eka-boron, eka-aluminium, 

 and eka-silicon, since named scandium, gallium, and 



germanium, by their discoverers, Cleve, Lecoq de Bois- 

 baudran, and Winckler. 



It might have been supposed that our knowledge of the 

 elements w-as practically complete ; that perhaps a few 

 more might be discovered to till the outstanding gaps in 

 the periodic table. True, a puzzle existed, and still exists, 

 in the classification of the " rare earths," oxides of metals 

 occurring in certain minerals ; these metals have atomic 

 weights between 139 and 1S0, and their properties preclude 

 their arrangement in the columns of the periodic table. 

 Besides these, the discovery of the inert gases of the atmo- 

 sphere, of the existence of which Johnstone Stoney's spiral 

 curve, published in 1888, pointed a forecast, joined the 

 elements like sodium and potassium, strongly electro- 

 negative, to those like fluorine and chlorine, highly electro- 

 positive, by a series of bodies electrically as well as 

 chemically inert, and neon, argon, krypton, and xenon 

 formed links between fluorine and sodium, chlorine and 

 potassium, bromine and rubidium, and iodine and caesium. 



Including the inactive gases, and adding the more 

 recently discovered elements of the rare earths, and 

 radium, of which I shall have more to say presently, there 

 are eighty-four definite elements, all of which find places 

 in the periodic table if merely numerical values be con- 

 sidered. Between lanthanum, with atomic weight 139, and 

 tantalum, 181, there are in the periodic table seventeen 

 spaces ; and although it is impossible to admit, on account 

 of their properties, that the elements of the rare earths can 

 be distributed in successive columns (for they all resemble 

 lanthanum in properties), yet there are now fourteen such 

 elements ; and it is not improbable that other three will 

 be separated from the complex mixture of their oxides by 

 further work. Assuming that the metals of the rare earths 

 fill these seventeen spaces, how many still remain to be 

 filled? We will take for granted that the atomic weight 

 of uranium, 238-5, which is the highest known, forms an 

 upper limit not likely to be surpassed. It is easy to 

 count the gaps ; there are eleven. 



But we are confronted by an embarras de richesse. The 

 discoverv of radio-activity by Henri Becquerel, of radium 

 by the Curies, and the theory of the disintegration of the 

 radio-active elements, which we owe to Rutherford and 

 Soddy, have indicated the existence of no fewer than 

 twenty-six elements hitherto unknown. To what places in 

 the periodic table can they be assigned? 



But what proof have we that these substances are 

 elementary? Let us take them in order. 



Beginning with radium, its salts were first studied by 

 Madame Curie ; they closely resemble those of barium — 

 sulphate, carbonate, and chromate insoluble ; chloride and 

 bromide similar in crystalline form to chloride and bromide 

 of barium ; metal, recently prepared by Madame Curie, 

 white, attacked by water, and evidently of the type of 

 barium. The atomic weight, too, falls into its place ; as 

 determined by Madame Curie and by Thorpe, it is 89-5 

 units higher than that of barium ; in short, there can be 

 no doubt that radium fits the periodic table, with an 

 atomic weight of about 2265. It is an undoubted element. 



But it is a very curious one. For it is unstable. Now, 

 stability was believed to be the essential characteristic of 

 an element. Radium, however, disintegrates — that is, 

 changes into other bodies, and at a constant rate. If 

 1 gram of radium is kept for 1760 years, only half a gram 

 will be left at the end of that time ; half of it will have 

 given other products. What are they? We can answer 

 that question. Rutherford and Soddy found that it gives 

 a condensable gas, which they named " radium emana- 

 tion " ; and Soddy and I, in 1903, discovered that, in 

 addition, it evolves helium, one of the inactive series of 

 gases, like argon. Helium is an undoubted element, with 

 a well-defined spectrum ; it belongs to a well-defined series. 

 And radium emanation, which was shown by Rutherford 

 and Soddy to be incapable of chemical union, has been 

 liquefied and solidified in the laboratory of University 

 College, London ; its spectrum has been measured, and its 

 density determined. From the density the atomic weight 

 can be calculated, and it corresponds with that of a 

 congener of argon, the whole series being: helium, 4; 

 neon. 20; argon, 40: krypton, 83: xenon. 130: unknown, 

 about 17S ; and niton (the name proposed for the emana- 

 tion to recall its connection with its congeners and its 



NO. 2183, VOL. 87] 



