WHAT IS AN ELEMENT ? 157 



elements ; and the radium gas shines in the dark, so that 

 a tube containing it gives off a whitish phosphorescent 

 light like that given off by stale fish, or like the luminosity 

 of the sea on calm summer evenings, or like the head of a 

 lucifer match if it is gently rubbed in the dark. If the 

 gas from radium is mixed with air, it is possible to see it 

 passing through a tube in the dark, and to recognise it by 

 its faint shining when it is transferred from one glass tube 

 to another. 



It is very easy to remove oxygen from a mixture of 

 gases ; if a piece of the element phosphorus be heated in 

 oxygen, a solid compound of the two is formed, and all 

 oxygen can then be got rid of; or oxygen may be ab- 

 sorbed by passing the mixed gases over red-hot copper. 

 Hence it is convenient to allow the emanation from 

 radium salts to mix with oxygen rather than with air; 

 for nitrogen, the other constituent of air, is more diffi- 

 cult to remove. And it is then possible to collect the 

 radium emanation, mixed with oxygen, in a glass tube, 

 and then to absorb the oxygen, leaving only the emana- 

 tion present. 



Now, as has been said, the emanation gradually loses its 

 power of discharging an electroscope. After four days it 

 requires twice as much emanation to produce the same 

 discharging effect as would be required if the emanation 

 were freshly prepared from radium salts. And the ques- 

 tion suggested itself to Mr. Soddy and Sir William Ramsay : 

 What becomes of the emanation ? Does it merely lose its 

 luminosity and discharging power, or is it changed into 

 something else ? 



Chemists have long had at their disposal a means of 

 recognising almost inconceivably minute quantities of 

 matter. All substances, when made into a gas by intense 

 heat, give out light ; and that light, if passed through a 

 prism, is seen not often to be all of one kind. For example, 



