78 SCIENCE PROGRESS 



with the radio-element disintegrating, and, in some cases, is 

 greater than 12,000 miles per second, a speed too great to be 

 conceived in the abstract, but which means half-way round 

 the world in one second ! One such case, for example, is that 

 of the transient element Radium C', the average life period of 

 which is only about one-millionth part of a second. 



The yS particle, on the other hand, is an electron, a particle 

 of disembodied electricity, or electricity divorced from matter, 

 of definite and constant negative charge, the unit to which 

 reference is made in the preceding paragraph. The velocity 

 of these particles also varies with the element disintegrating, 

 and in some cases is no less than that of light itself, 186,000 

 miles per second, or more than seven times round the world 

 in a second. The velocity of the /9 rays from many elements is 

 considerably less than this, varying from 40 to 80 per cent, of 

 it, values that are still enormous. 



The 7 rays are X-rays of shorter wave-length, and therefore 

 greater frequency, than those produced artificially, vibrations 

 of the ether several thousand times shorter than those of light, 

 set up by the movement of electrons in the innermost region of 

 the atom. 



The speed of all these rays is so great that it gives them the 

 power of penetrating the atom, and going right through it. 

 Their penetrating power may vary considerably, yet that of one 

 class is quite distinct from that of the others, and by this 

 means the rays were first classified. Thus the most penetrating 

 a ray known is only about twice as powerful as the least pene- 

 trating, but the penetrating power of the ^ rays as a whole 

 is about one hundred times that of the a rays as a whole, and 

 that of the 7 rays in turn is one hundred times that of the /3 

 rays. 



Now as the a and /S particles penetrate the atom, they 

 will sometimes pass very close indeed to the nucleus or the 

 electrons and thereby be deflected from their course, the y8 

 particles fairly easily because they are so light. This can be 

 shown by allowing a parallel beam of ^ rays to fall on a thin 

 sheet of a substance, when it will be noticed that the emerging 

 beam is no longer parallel, but scattered. 



C. T. R. Wilson has succeeded in photographing the paths 

 of these particles in air and in other gases. Those of the a 

 rays are almost perfectly straight, but with an occasional large 

 deflection, for the a particles are so massive, compared with 

 the electrons, that it requires the cumulative effect of a large 

 number of the latter to produce any appreciable spreading of 

 an a beam, or the action of the nucleus of the atom to deflect 

 an a particle. Experiments based on these deflections have 

 shown that the positive charge of the nucleus of the atom, 



