RADIUM AND ITS PRODUCTS 185 



helium had accumulated. But whence came the helium ? 

 That was the next question to be settled. 



A solution of radium bromide gives off gas continuously. 

 That gas, on investigation, is found to be a mixture of 

 oxygen and hydrogen, the constituents of the water in 

 which the bromide is dissolved. It contains, however, a 

 small excess of hydrogen, which implies that some oxygen 

 has been absorbed, probably by the radium bromide, 

 although what becomes of that excess has not yet been 

 determined. 



When an electric spark is passed through a mixture of 

 oxygen and hydrogen, an explosion takes place; the 

 gases combine, and water is formed. Any excess of 

 hydrogen is, however, unaffected. Now the gases evolved 

 from a solution of radium bromide make glass luminous in 

 the dark, and possess the power of discharging an electro- 

 scope, like radium bromide itself. Rutherford and Soddy 

 discovered that when this mixture of gases is led through 

 a tube shaped like a U, cooled to - 185 C. by dipping in 

 liquid air, the luminous gas condenses, and the gases 

 which pass on have nearly ceased to be luminous in 

 the dark, and no longer discharge an electroscope. To 

 such condensable gases Rutherford applied the term 

 ' emanation ' ; this one is known as the ' radium emana- 

 tion.' 



The next question to be answered was : Is the helium 

 evolved from the radium bromide directly, or is it a pro- 

 duct of the emanation ? It was necessary, therefore, to 

 collect the emanation and to examine its spectrum. This 

 was managed, after many unsuccessful trials, by exploding 

 the mixture of oxygen and hydrogen containing the 

 emanation, allowing the remaining hydrogen to pass into 

 a tube containing a thin spiral of slightly oxidised copper 

 wire kept at a red heat by an electric current: the 

 hydrogen combined with the oxygen of the oxide of 



