THE TRANSFORMATION OF ELEMENTS 73 



nation in any time was estimated by the increase of activity of 

 the gas into which it diffused. Since in the argon series the 

 molecular and atomic weights appear to be identical, their 

 evidence goes to show that the atomic weight of the emanation 

 is of the order 200. 



On the assumption that the emanation occupies a space in 

 the periodic series twice as much above xenon as xenon is above 

 krypton, the intermediate space corresponding to the figure 173 

 being unfilled, Ramsay has calculated that its atomic weight 

 should be 216*5. If an a particle consists of a helium atom, 

 and if the atomic v/eight of radium is 226*5, then, since radium 

 and its emanation appear to be consecutive products, the 

 atomic weight of the latter should be 222*5. Rutherford and 

 Soddy found that the emanation condenses to liquid or solid 

 form at about — 155 C. Henriot states that the solidified 

 emanation commences to volatilise between — 150° and — 160 , 

 at a few millimetres pressure only. It should be pointed out 

 that this figure by no means agrees with the supposition that 

 it is a gas of the argon series of atomic weight about 220. 

 Xenon (atomic weight 128) melts at —140°, and by extrapolation 

 the value for an element of the series of atomic weight 220 should 

 be in the neighbourhood of — 6o°. 



Radium emanation can be readily condensed by the use of 

 liquid air. In order to show how it is isolated, and its pro- 

 perties examined, a somewhat detailed account will now be 

 given of the methods employed by Ramsay, in his researches 

 on this gas with Collie, and with Soddy and Cameron. 



The radium salt is dissolved in water in a small bulb a 

 (fig. 1), connected through the trap b with the intake tube of 

 a mercury Topler pump. The tap c prevents the mercury 

 flowing back into the bulb, the trap b catching any that 

 is accidentally allowed to pass the tap. The bulb and tubes 

 are initially pumped empty of air ; the mercury is forced past 

 the valve d (by raising the reservoir e sufficiently and closing 

 the outlet tube f) and a little way past c, which is then closed. 

 The mercury seal entirely prevents leakage through c. The 

 emanation is allowed to collect during three or four days, and 

 is then pumped off by repeatedly lowering the reservoir e, 

 opening c for a few seconds, and forcing the gas which passes c 

 through the outlet tube f into the test-tube g, previously filled 

 with, and inverted over, mercury. The operation is continued 



