190 



RADIUM. 



and directed toward the r?ar of this plane, the /? ra3's are strongly 

 deflected toward the right and caused to follow a circular trajectory; 

 the a ra3\s are slig-litl}' deviated toward the left, while the y rays, 

 which are far the least abundant, continue in a straight line. 



The a rays are of slight penetrating power. A sheet of aluminum 

 only a few hundredths of a millimeter in thickness absorbs them. To 

 exhibit their deviation a very intense magnetic field is required, and 

 the actual demonstration requires a far more delicate method than 

 that indicated in lig. 2, which is merely a diagram given in general 

 illustration." 



The Of ra3's may 1)(^ compared to projectiles of atomic dimensions 

 charged with positive electricity and shot oti' with great velocities. 

 Apart from tlicir behavior in a magnetic field, the ot rays may be rec- 

 ognized by their manner of absorption in a succession of thin screens.* 

 In traversing successively a series of screens the (y ra3's become less 



and less penetrating, whereas 

 residual Kontgen I'ays under 

 these circumstances ))ecome 

 more and more penetrating. 

 It app(\ars that the energy of 

 ciich projectile becomes less 

 1 l a Q with each screen that it passes. 



Or I It is the ft" rays which appear 



to l)e active in the ))eautiful 

 experiment exhibited l>y the 

 spinthariscope of Sir AVilliam 

 C'rookes. In this apparatus a 

 small fragment of radium salt 

 (only a fraction of a milligram) 

 is suspended ])y a metallic wire 

 at a small distance (one-half 

 millimeter) from a screcMi of phosphorescent sulphide of zinc. When 

 the face of the screen whicii is turned toward the radium is examined 

 in darkness by the aid of a magnifying glass, it appears studded with 

 sparkling points, reminding one of the stars in the sky, except that 

 these huninous points are appearing and disappearing continuall3\ 

 It ma}' l)e supposed that each l)right point which appears is the result 

 of the impact of a projectile, and thus for the first time there has been, 

 discovered a means of distinguishing an individual action of an atom. 

 The /3 rays are analogous to the cathode rays and behave similarl}^ 

 in the magnetic field. They comi)()rt themselves as projectiles charged 

 negatively' and escaping from the radium with high velocity. These 

 projectiles (electrons) appear to have a mass about one thousand times 



« Rutherford, I'liil. Majj;., P>brmiry, 190.'i Becquerel, C. R., Vol. CXXXVI, p. 

 199. 



&Mme. Curie, C. R., .lanuary s, IHOO. 



!■ 



h 



is: E 



Fui. 3. — Magnt'tif deviation of p rny> 



