RADIUM AND ITS PRODUCTS 181 



may not be connected with their radioactivity.' Besides 

 the premises already mentioned, they had evidence of 

 the probable mass of the ' a-particles,' which appeared to 

 be about twice that of an atom of hydrogen. Now 

 helium is the lightest gas next to hydrogen; and its 

 atoms are four times as heavy as atoms of hydrogen. It 

 was, therefore, a striking confirmation of the accuracy of 

 this view when Ramsay and Soddy discovered that helium 

 can actually be obtained from radium. 



Before giving an account of that discovery, a short 

 description of the nature and properties of helium may 

 not be out of place. When light passes through a prism, 

 it is refracted or bent ; and Newton discovered that white 

 light, such as is emitted from the sun or the stars, after 

 passing through a prism, gives a spectrum consisting of 

 coloured images of the hole in the window-shutter through 

 which the sunlight fell on his prism. Fraunhofer, a 

 Berlin optician, conceived the idea of causing the light to 

 pass through a narrow slit, instead of a round hole ; and 

 the spectrum then consisted of a number of images of the 

 narrow slit, instead of the round hole. He was struck by 

 one peculiarity shown by sunlight when thus examined, 

 namely, that the coloured band, rainbowlike, and exhibit- 

 ing a regular gradation of colour from red at the one end, 

 through orange, yellow, green, and blue, to violet at the 

 other end, was interspersed by very numerous thin black 

 lines. The nature of these lines was discovered by 

 Kirchoff. The light emitted by a white-hot body shows 

 a continuous spectrum ; but if such white light be passed 

 through the vapours of a metal, such as sodium, a portion 

 is absorbed. For example, glowing sodium gas shows two 

 yellow lines, very close together ; but if this light is passed 

 through the vapour of sodium, these lines are extinguished 

 if the correct amount of vapour be interposed. Now it 

 was found that the position of the two dark lines in the 



