106 



THE NEW KNOWLEDGE. 



its velocity (Energy^-); and hence the energy carried 



off from radium by these little flying bullets is almost in- 

 credibly large compared with their mass. As Professor 

 Rutherford says: "If it were possible to give an equal ve- 

 locity to an iron cannon ball, the heat generated by the im- 

 pact on a target would be many thousand times more than 

 sufficient to melt the cannon ball and dissipate it into 

 vapour." This is the conclusion of mathematical reasoning, 

 but no amount of reasoning is half so demonstrative to the 

 average mind as the sight of a fragment of radium at work. 

 We have stated that the mere presence of radium will cause 

 certain substances in the neighbourhood to phosphoresce or 

 to light up with a glow. Now among the substances af- 

 fectable by radium there is one, zinc sulphide, which answers 



to the bombardment of the 



B 



T 



Fig. 35. 



alpha-rays alone. This 

 property has been utilized 

 by Sir William Grookes in 

 the construction of a little 

 instrument which he has 

 called a sphinthariscope, 

 which shows beautifully the 

 bombardment of the alpha- 

 rays. This instrument con- 



Section of Crookes' Spinthariscope, sists of a smallJ^agnient of 

 T The outer containing tube. radium placed behind a zinc 



T,T The sliding tube, for focussing. su i p hide screen, which is 



A The screen, coated inside with . . . . . . 



blende, viewed in the dark through 



B The wire supporting the speck a tube containing a magni- 



of radium salt. f in eyepiece (Fig. 35) . At 



C The magnifying lens. J 



once one sees a thousand tiny 

 flashes of light, for the spark of each atomic projectile as 





