728 Condensation of the Radium Emanation* 



with time. Viewed with the naked eye, the end o£ the 

 capillary containing the liquid at first appears rose-coloured, 

 but in the course of an hour or two becomes greenish. This 

 is no doubt due to the increased local phosphorescence of the 

 glass, due to the radiation from the active deposit formed by 

 the condensed emanation. If the emanation is volatilized its 

 colour instantly disappears, showing that the colour is a 

 property of the liquid or solid emanation at low temperature 

 when bombarded by its own a particles. 



The extreme end of the capillary where it had been drawn 

 off was conical in shape. With care, the whole of the liquid 

 could be concentrated in this glass cone. This was most 

 simply done by applying a pad of cotton-wool soaked in 

 liquid air to the end of the capillary. The moment the 

 emanation was all condensed, the capillary was plunged into 

 liquid air. Under these conditions, the volume of the 

 capillary occupied by the liquid could be examined at leisure 

 by a microscope. The colour of the condensed emanation 

 made it comparatively easy to locate its distribution. After 

 several trials, the whole of the emanation was liquefied in the 

 extreme tip of the capillary and did not occupy a length of 

 more than a fifth of a millimetre. Knowing the diameter 

 of the capillary the volume occupied by the liquid could be 

 estimated approximately. This volume was certainly not 

 greater than 1*2 x 10 -4 cubic millimetre. 



The amount of emanation in the capillary corresponded to 

 100 milligrams of radium, and was consequently equal to 

 about *06 c.mm. We thus see that the volume of the liquid 

 emanation in liquid air was certainly not greater than 1/500 of 

 the volume of the gas at normal pressure and temperature. 

 Taking the emanation as a monatomic gas of atomic weight 

 222, it can readily be calculated from the above data that the 

 density of liquid emanation is not less than 5. No doubt if a 

 much finer capillary were used in which to condense the 

 emanation, a more accurate estimate could be made. 



It has so far not been found possible to determine directly 

 the density of the emanation on account of the very small 

 quantity available for experiment. Its atomic weight can, 

 however, be deduced with considerable confidence from 

 radioactive considerations. It has recently been shown by 

 Rutherford and Royds * in a decisive experiment that the 

 a particle is an atom of helium. Since the atom of the 

 emanation is derived from the radium atom (atomic weight 

 226) by the expulsion of ana particle, its atomic weight should 

 be 222. The absence of combining properties of the emanation 



Phil. Mag. Feb. 1909. 



