§ =89.= Radium salts give off three distinct sorts of rays, referred to by the Greek letters α, β, γ. The α-rays have been shown to consist of electrically charged (positive) particles, with a mass approximately equal to that of four hydrogen atoms; they are slightly deviated by a magnetic field, and do not possess great penetrative power. The β-rays are similar to the kathode rays, and consist of (negative) electrons; they are strongly deviated by a magnetic field, in a direction opposite to that in which the α-particles are deviated, and possess medium penetrative power, passing for the most part through a thin sheet of metal. The γ-rays resemble X-rays; they possess great penetrative power, and are not deviated by a magnetic field. The difference in the effect of the magnetic field on these rays, and the difference in their penetrative power, led to their detection and allows of their separate examination. Radium salts emit also an emanation, which tends to become occluded in the solid salt, but can be conveniently liberated by dissolving the salt in water, or by heating it. The emanation exhibits the characteristic properties of a gas, it obeys Boyle’s Law (_i.e._, its volume varies inversely with its pressure), and it can be condensed to a liquid at low temperatures; its density as determined by the diffusion method is about 100. Attempts to prepare chemical compounds of the emanation have failed, and in this respect it resembles the rare gases of the atmosphere--helium, neon, argon, krypton, and xenon--whence it is probable that its molecules are monatomic, so that a density of 100 would give its atomic weight as 200.[105] As can be seen from the table on pp. 106, 107, an atomic weight of about 220 corresponds to a position in the column containing the rare gases in the periodic system. That the emanation actually has an atomic weight of these dimensions was confirmed by further experiments carried out by the late Sir William Ramsay and Dr. R. W. Gray.[106] These chemists determined the density of the emanation by actually weighing minute quantities of known volume of the substance, sealed up in small capillary tubes, a specially sensitive balance being employed. Values for the density varying from 108 to 113½, corresponding to values for the atomic weight varying from 216 to 227, were thereby obtained. Sir William Ramsay, therefore, considered that there could no longer be any doubt that the emanation was one of the elements of the group of chemically inert gases. He proposed to call it _Niton_, and, for reasons which we shall note later, considered that in all probability it had an atomic weight of about 222½. [105] This follows from Avogadro’s Hypothesis, see § 76. [106] Sir WILLIAM RAMSAY and Dr. R. W. GRAY: “La densité de l’émanation du radium,” _Comptes Rendus hebdomadaires des Séances de l’Académie des Sciences_, vol. cvi. (1910), pp. 126 _et seq._ The Disintegration of the Radium Atom.