1002 



SCIENCE. 



[N. S. Vol. XVII. No. 443. 



mass by reason of its electrodynamie prop- 

 erties, and if we consider all forms of mat- 

 ter to be merely congeries of electrons, the 

 inertia of matter would be explained with- 

 out any material basis. On this view the 

 electron would be the 'protyle' of 1886, 

 whose different groupings cause the genesis 

 of the elements. 



There is one more property of the ema- 

 nations of radium to bring before your 

 notice. I have shown that the electrons 

 produce phosphorescence of a sensitive 

 screen of barium platinocyanide, and the 

 positive ions of radium produce phosphor- 

 escence of a screen of zinc blende. 



If a few minute grains of radium salt 

 fall on the zinc sulphide screen the surface 

 is immediately dotted with brilliant specks 

 of green light. In a dark room, under a 

 microscope with a two-third-inch objective, 

 each luminous spot shows a dull center 

 surrounded by a diffused luminous halo. 

 Outside the halo the dark surface of the 

 screen scintillates with sparks of light. No 

 two flashes succeed oh the same spot, but 

 are scattered over the surface, coming and 

 going instantaneously, no movement of 

 translation being seen. 



If a solid piece of a radium salt is 

 brought near the screen, and the surface 

 examined with a pocket lens magnifying 

 about 20 diameters, scintillating spots are 

 sparsely scattered over the surface. Bring- 

 ing the radium nearer the screen the scin- 

 tillations become more numerous and 

 brighter, until when close together the 

 flashes follow so quickly that the surface 

 looks like a turbulent luminous sea. When 

 the scintillating points are few there is no 

 visible residual phosphorescence, and the 

 successive sparks appear ' atoms of intensest 

 light, ' like stars on a black sky. What to 

 the naked eye seems like a uniform 'milky 

 way,' under the lens becomes a multitude 



of stellar points, flashing over the whole 

 surface. 



'Polonium' basic nitrate, actinium and 

 radio-active platinum produce a similar 

 effect on the screen, but the scintillations 

 are fewer. In a vacuum the scintillations 

 are as bright as in air, and being due to 

 inter-atomic motion they are not affected 

 by extremes of low temperature : in liquid 

 hydrogen they are as brilliant as at the 

 ordinary temperature. 



A convenient way to show these scintil- 

 lations is to fit the blende screen at the 

 end of a brass tube with a speck of radium 

 salt in front about a millimeter off, and 

 to have a lens at the other end. I propose 

 to call this little instrument the ' spinthari- 

 scope,' from the Greek word a-Ki'^da.pL'z * a 

 scintillation. 



It is difficult to estimate the number of 

 flashes of light per second. With the ra- 

 dium about five centimeters off the screen 

 the flashes are barely detectable, not more 

 than one or two per second. As the dis- 

 tance of the radium diminishes, the flashes 

 become more frequent, until at one or two 

 centimeters, they are too numerous to count, 

 although it is evident this is not of an 

 order of magnitude inconceivably great. 



Practically the whole of the luminosity 

 on the blende screen, whether due to ra- 

 dium or 'polonium,' is occasioned by ema- 

 nations which will not penetrate card. 

 These are the emanations which cause the 

 scintillations, and the reason why they 

 are distinct on the blende and feeble on 

 the platinocyanide screen, is that with the 

 latter the sparks are seen on a luminous 



cLGTEpi elddfjsEiJo^j fiEOu ^fiarl rov 6' cltto TroA/lat 

 aTnvdapi6e( narCyvTo, atTiag (5' t'lq ovpavbv \kcv\ 



(Here from the ship leaped the far-darting 

 Apollo, like a star at midday, while from him 

 flitted scintillations of fire, and the brilliancy 

 reached to heaven. ) Homer's ' Hymen to Apollo,' 

 lines 440-442. 



