RADIUM AND ITS PRODUCTS 183 



elements uranium and thorium, when heated, or when 

 treated with acids, gave off a gas which he took for 

 nitrogen. But the discovery of argon had taught Ramsay 

 how to deal with such a gas. He examined it in the hope 

 that it might lead to the discovery of a compound of 

 argon ; but its spectrum turned out to be identical with 

 that of solar helium, and terrestrial helium was discovered. 

 It proved to be a very light gas, only twice as heavy as 

 hydrogen, the lightest substance known ; its spectrum con- 

 sists chiefly of nine very brilliant lines, of which D 3 is the 

 most brilliant ; it has never been condensed to the liquid 

 state, and is the only gas of which that can now be said 

 (for hydrogen has been liquefied within the last few years 1 ), 

 and, like argon, it has not been induced to form any 

 chemical compound. That it is an element is shown by 

 the relation of its atomic weight, 4, to that of other 

 elements, as well as by certain of its properties, the most 

 important of which is the ratio between its specific heat 

 at constant volume and constant pressure ; but to explain 

 the bearing of this property on the reasoning which proves 

 it to be an element would be foreign to the subject of this 

 article. 



This then was the elementary substance that Ruther- 

 ford and Soddy suspected to be one of the decomposition 

 products of radium. The word ' decomposition,' however, 

 implies the disruption of a compound, and the change 

 which takes place when radium produces helium is of 

 such a striking nature that it is perhaps preferable to use 

 the term c disintegration.' 



Having procured fifty milligrammes (about three- 

 quarters of a grain) of radium bromide, Ramsay and 

 Soddy placed the greyish-brown crystalline powder in a 

 small glass bulb about an inch in diameter. This bulb 

 was connected by means of a capillary tube with another 



1 It has since been liquefied by Kammerlingh Onnes of Leiden. 



