710
C H E M I S T R Y
closely resembles argon, being also an entirely inert gas; its density is only about twice that of hydrogen (cf. Proe. Chem. Soc. 1895, p. 59; Trans, ditto, 1895, p. 1107; Proc. Roy. Soc. 60, p. 442). Helium has since been extracted from a variety of minerals consisting Helium, 0f sajts 0f uranium, yttrium, thorium and other rare earths. It is present in the gas evolved from the Bath springs and in other natural waters. Dewar has shown that it may be separated from the Bath spring gas by subjecting this to a process of fractional liquefactions by means of liquid air {Proc. Chem. Soc. 1897, p. 187). By adopting such a method, as well as by the fractional distillation of liquefied argon and liquefied air, Ramsay and Travers have more recently succeeded in obtaining evidence of the presence in excessively minute proportions of three other inert gases besides helium and argon in air; they have named these neon, krypton, and xenon. In the account of their work laid before the Royal Society on 30th November 1900, the relative density of neon was stated to be nearly 10 (9'97), that of krypton 40-88, and that of xenon 64. The quantities of the three gases dealt with were so small, however, that these values must be regarded as provisional. When vacuum tubes charged with the gases are excited, that containing neon assumes an intense flame colour, the krypton tube a pale violet, and the xenon tube a sky blue. Neon can be liquefied only with the aid of liquid hydrogen. Krypton meltsat — 169° and boils at about — 152°, whilst xenon melts at — 140° and boils at — 109°; the melting-point of argon, it may be mentioned, is - 188°, and its boiling-point - 186-4°. They appear to resemble helium and argon in properties. According to Liveing and Dewar {Proc. Roy. Soc. 1901, 67, p. 467), the spectra of the more volatile gases of atmospheric air and of the Bath spring gas which are not condensed at the temperature of liquid hydrogen contain numerous lines occupying positions very near to those assigned to lines in the spectra of the solar corona and of the aurora. It may, therefore, be anticipated that yet other elements will be discovered in the air. Shortly after the discovery of the Rontgen rays, the discovery, if possible more remarkable, was made by Henri Becquerel that uranium salts have the power of active' emitting rays capable—even after traversing elements. thin sheets of glass, ebonite, and metals—of producing photographic effects, as well as of rendering air a conductor of electricity. Thorium compounds were subsequently found to emit similar rays. The early experiments appeared to show that the intensity of the effect was proportional to the amount of uranium present, the metal being more active than any of its salts; but it was then found that certain uranium minerals— pitchblende and chalcolite—were more active even than the metal. Led by this fact, M. and Mme. Curie undertook the investigation of these materials, and succeeded in showing that at least two radio-active “ elements ” are present in them. One of these, polonium, occurs along with bismuth in the precipitate produced by sulphuretted hydrogen in a solution of the mineral in acid. The other, radium, occurs in the barium chloride prepared from pitchblende. Barium chloride so obtained is not only characterized by gi’eat radio-activity, but also affords a spectrum in which there are a number of lines not hitherto noticed in the spectrum of any element. Moreover, according to Mme. Curie, the atomic weight of radio-active “ barium ” increases as the intensity of the radiant effect increases, a substance exerting an activity 3000 times that of uranium giving the value 140, while in one exerting an activity 7500 times as great the value is as high as 145-8. Her latest determination, made with a substance which, from its spectrum, appeared to contain rather more
radium than barium, gave the value 174. Quite recently {Proc. Roy. Soc. 1900, p. 409) Crookes has shown that the uranium salts of commerce may be purified until free from radio-activity, one method being to re-crystallize uranium nitrate from ether, in which it is readily soluble. Crookes proposed to term the radio - active constituent — which becomes concentrated in the residue—provisionally Ux. It closely resembles “ radium,” but its exact nature remains to be determined. According to Crookes, polonium is distinguished from radium by the inability of the rays which it emits to penetrate glass. Radium compounds are self-luminous even after they have been kept for months in the dark. Hitherto only uranium and thorium minerals have been observed to exhibit radio-activity, and apparently the property is common to all of these. As a small proportion of radium raises the atomic weight of barium considerably, it is evident that the “ element ” must have a high atomic weight; its constant and exclusive association in nature with the two elements of highest atomic weight known is therefore particularly remarkable.1 It will be noticed that in the case of the rare-earth metals, the elements from air and the radio-active elements, the discovery of new elements has frequently been consequent on the introduction of novel methods rather than on the discovery of new materials : in fact, in only one case, that of germanium, has a previously untouched material been dealt with. Classification of Elements. Before discussing the bearing of the new discoveries of elements, it is necessary to consider briefly the rectifications effected in our knowledge of atomic weights within recent years. It must be remembered that although atomic weight is the factor of primary importance in the classification of the elements from the point of Qectificaview of the periodic law, it can obviously only tion of serve as a basis of classification in cases in atomic which it is clear that the value has been ascer- we'£htstained with a sufficiently close approach to accuracy. It is therefore one of its chief recommendations that the law itself affords a means of correcting atomic weights ; indeed its acceptance as a valid generalization has been most materially promoted by the manner in which Mendeleeff’s early proposals to alter the values assigned to a number of the rarer elements have been justified. To understand this, it is only necessary to bear in mind that when the elements are arranged in the order of atomic weight, instead either of an irregular or of a continuous variation in properties, a more or less obviously regular periodic variation is noticed—a given property coming to a maximum, then gradually diminishing to a minimum, from which point it again rises to a maximum. Consequently, at intervals throughout the series, elements are met with having similar properties. These elements serve as guideposts, and in constructing a table are placed in corresponding positions. This principle being recognized, it is possible to locate correctly elements of which the atomic weights are but approximately known. Thus osmium, iridium, and platinum were at the outset placed by Mendeleeff in advance of gold, although their then accepted weights were superior to that of gold, for the reason that the other platinum metals, as well as iron, cobalt and nickel, came in advance of the superior terms of the family in which gold is included. Determinations made by Seubert completely justified Mendeleeff’s action, while 1
According to Debierne, pitchblende contains a radio-active “element” of the iron group, resembling thorium in its properties— actinium. Hofmann has separated from a number of rare - earth minerals an element resembling lead, but distinguished by radioactivity, the equivalent of which is about 65, and the atomic weight perhaps about 260.
