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 } Yb 172.0 (Neoytterbium) } 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.