Thus, too, in the case of the nitrogen group:
| Nitrogen. | Phosphorus. | Arsenic. | Antimony. | Bismuth. | |
|---|---|---|---|---|---|
| 14 | 31 | 75 | 119 | 207 | |
| a | a + d | a + d + d´ | a + d + 2d´ | a + d + 4d´ |
where a = 14; d = 17; d´ = 44.
* * * * *
On the basis of these and similar numerical relationships it was surmised that, just as the successive members of a group of homologous organic radicals are formed by increments of CH2, so the substances in the several groups of the elements may be produced by successive additions of some form of matter common to them all. This has its counterpart, somewhat modified, in the modern hypothesis of the disintegration of the elements. Dumas conceived the elements in any particular group to be built up by successive accretions of particular forms of matter; Rutherford and Soddy suppose them to be derived by the successive elimination of matter from some unstable parent substance.
Since 1850 the existence of at least twenty-two new elements may be said to have been established. Of course, many more than this number have been announced, more or less tentatively; but subsequent investigation has either not confirmed their existence, or has definitely disproved it. The names, symbols, and atomic weights of the twenty-two, arranged in alphabetical order, are as follows:
| Argon | A | 39.9 | |
| Cæsium | Cs | 132.8 | |
| Dysprosium | Dy | 162.5 | |
| Europium | Eu | 152.0 | |
| Gadolinium | Gd | 157.3 | |
| Gallium | Ga | 69.9 | |
| Germanium | Ge | 72.5 | |
| Helium | He | 4.0 | |
| Indium | In | 114.8 | |
| Krypton | Kr | 83.0 | |
| Lutecium | Lu | 174.0 | |
| Neodymium | Nd | 144.3 | |
| Neon | Ne | 20.0 | |
| Praseodymium | Pr | 140.6 | |
| Radium | Ra | 226.4 | |
| Rubidium | Rb | 85.4 | |
| Samarium | Sa | 150.4 | |
| Scandium | Sc | 44.1 | |
| Thallium | Tl | 204.0 | |
| Thulium | Tm | 168.5 | |
| Xenon | Xe | 130.7 | |
| Ytterbium (Neoytterbium) | } | Yb | 172.0 |
The additions have been due, to some extent, to the refinement of processes of analysis already in use, but more especially to the employment of new analytical methods; or, lastly, to the application of a generalisation concerning the mutual relations of the elements which has served to indicate not only the existence of new and specific members of families of elements already known, but to point out the probable mode of their occurrence.[2]
[2] The substances which appear to be formed by the disintegration of uranium, radium, thorium—the so-called radio-active elements—such as ionium, actinium, polonium, and the various emanations to which they give rise, are not here enumerated. They are dealt with in Chapter III.
Although the existence of the element fluorine was surmised as far back as 1771, when Scheele first recognised that the product of the action of oil of vitriol upon fluor-spar contained a hitherto unknown substance, it was not until 1886 that this substance was definitely isolated by Moissan by the electrolysis of the acid potassium fluoride in solution in hydrogen fluoride. Cerium tetrafluoride, CeF4, and lead tetrafluoride, PbF4, when heated, were observed by Brauner to evolve a gas having a smell resembling that of hypochlorous acid, which was probably free fluorine. Certain violet-coloured varieties of fluor-spar, when powdered, emit a peculiar smell, which has been attributed to free fluorine.