198 BIOGRAPHIES OF SCIENTIFIC MEN 



whose vision they lock up in silence, content with the 

 work of the day." 



At the 1903 meeting of the British Association, Kelvin 

 contributed a paper on the subject, of which the following 

 is a resume: Radium has been found to emit three 

 types of rays (1) the a rays, positively electrified and 

 largely stopped by solid, liquid, or gaseous screens ; 

 (2) ft rays, more penetrative than a and negatively elec- 

 trified ; (3) 7 rays, electrically neutral and much more 

 penetrative than either of the other two, passing with 

 but little loss through a lead screen one centimetre thick, 

 which is an almost perfect screen against the other rays. 

 A simple prima facie view was to regard the 7 rays as 

 mere vapour of radium ; the ft rays seem certainly to be 

 atoms of resinous electricity or electrons. The a rays 

 were atoms of molecules of matter, probably atoms of 

 radium, or perhaps molecules of radium bromide. The 

 electro-ethereal hypothesis afforded a ready explanation of 

 the relative penetrating power of the three radiations, 

 and of the fact that each one of them made its existence 

 known to us by conferring electric conductivity on air or 

 on any ordinary gas on which it was present. Taking 

 the 7 rays first, we had to explain the free penetration of 

 unelectrified radium molecules through dense liquid and 

 solid matter. An easy assumption sufficed. Let the 

 Boscovichian mutual forces that was, the chemical 

 affinities and the repulsions between an atom of radium 



