DISCOVERY AND PROPERTIES OF RADIUM 153 



by a thin sheet of mica. This seemed to agree with the properties 

 of a gas, and it was proved by special experiments that the 

 observed activity was not due to particles of dust. Further 

 investigation showed that the " emanation," as it is called, 

 possesses the properties of a chemically inert gas. This substance 

 has since been named " niton " by Sir William Ramsay, who, 

 notwithstanding its instability, places it among the argon group 

 of elements in the periodic scheme. 



According to Rutherford's disintegration theory of radio- 

 active change a definite number of atoms of radium break up 

 per second, each atom evolving an a particle which ultimately 

 becomes a helium atom, leaving behind the residue of the atom 

 which forms the gas known as the " emanation " or niton. It 

 seems to be agreed that the radium atom on disintegration to 

 niton splits up into two parts only, one of which is the a particle. 

 The atomic weight of the resulting niton must be therefore the 

 atomic weight of radium minus the atomic weight of helium or 

 2264-4=2224. 



This question, however, as Sir William Ramsay remarks, can 

 only be settled by appeal to experiment, and in 1910, in associa- 

 tion with Dr. Whytlaw Gray, and with the aid of a balance 

 constructed on the same principle as the balance used for the 

 estimation of the atomic weight of radium he proceeded to deter- 

 mine the density of niton. 1 To appreciate the extreme delicacy 

 and difficulty of the operations involved it is necessary to read 

 the original memoir in its entirety. It will be sufficient to state 

 in this place a few facts connected with the enquiry. 



To determine the density of a gas, four separate measure- 

 ments are essential, -the volume, the temperature, the pressure, 

 and the weight of the gas. In the present case the volume of 

 niton which accumulates in a given time from a known weight 

 of radium is a constant quantity and has been repeatedly 

 measured. In the present case the total volume of niton obtain- 

 able for weighing scarcely exceeded 0-1 cubic mm. The weight 

 of this volume on the assumption that the atomic weight is 

 222 is less than yifVo mgrm. It is therefore evident that in order 

 to weigh this minute quantity of gas with sufficient exactness a 

 balance turning with a load not greater than y^o THF nigrm. is a 

 necessity. This seems an almost inconceivably small weight to 



1 "The Density of Niton (Radium Emanation) and the Disintegration 

 Theory," Proc. Royal Soc., vol. 84 (1911), p. 536. 



