158 ESSAYS BIOGRAPHICAL AND CHEMICAL 



the light given out by sodium gas at a red heat is yellow ; 

 and if passed through a slit, and then through a prism, 

 two yellow lines are seen the spectrum of sodium. 

 Similarly, potassium salts, in a spirit-lamp flame, gives 

 out a violet light ; and the prism shows us that the light 

 consists of two kinds one red and one violet. And so 

 for other elements. If the spectra of gases have to be 

 examined, they can be made to glow by passing an electric 

 discharge through a very narrow tube containing a minute 

 trace of the gas. Helium, for example, if examined in 

 this way, gives out light consisting of many colours : red, 

 yellow the most intense green, green-blue, blue and 

 violet. Hence it is easy to recognise the presence of 

 helium in such a capillary tube, by passing an electric 

 discharge through it, for the exact position of the lines 

 in its spectrum is easily recognised. 



Now Ramsay and Soddy found that the emanation from 

 radium salts, though it gave out a special light of its own 

 when made luminous by an electric discharge, showed 

 none of the lines characteristic of helium. But after 

 standing for three days the yellow line of helium began to 

 be visible, and that is the one most easily seen. As time 

 went on, and as the emanation lost its self-luminosity, the 

 other lines denoting the presence of helium became dis- 

 tinctly visible. The conclusion was forced upon them, 

 therefore, that, as the emanation disappears, helium is 

 formed, or, in., other words, the emanation is changing 

 slowly into helium. 



Professor J. J. Thomson, of Cambridge, has of recent 

 years been investigating the motion of particles which are 

 shot off from the negative pole when an electric discharge 

 is passed through gases ; and he has succeeded in showing 

 that some of the particles move with enormous rapidity, 

 and that they possess a weight which cannot be much 

 more than one seven-hundredth of that of a hydrogen 



