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THE AMERICAN NATURALIST [Vol. XLII 



Rutherford involves a conception of the atom as a body 

 composed of intricately related units. These units pos- 

 sess relatively enormous amounts of kinetic energy, and 

 are in rapid orbital motion within the atom. In some 

 substances of high atomic weight, such as uranium, polo- 

 nium and radium, these units spontaneously escape from 

 the atom and fly off into space. Such substances are 

 called radioactive, and the emission of these units is 

 radioactivity. 



The particles themselves are called ions. They are of 

 at least two kinds ; one, called the p particles, very small 

 (about one one-thousandth the size of a hydrogen atom), 

 bearing a charge of negative electricity, and moving with 

 a velocity approaching that of light; the other called a 

 particles, about twice the size of a hydrogen atom, bear- 

 ing a positive electrical charge, and moving at a much 

 lower velocity than the p particles. The p particles or 

 negative ions are called electrons. 



Streams of negative electrons constitute the so-called 

 P rays ; streams of positive ions the a rays. Both a and 

 P particles move with velocities that vary between cer- 

 tain limits and so the respective rays are complex. 



In addition to the giving off of a and p rays, radioac- 

 tivity involves the emission of electro-magnetic pulses 

 in the ether. These are analogous to very penetrating 

 X rays, and are called y rays. 2 



The enormous velocity of the P particles, combined with 

 their inconceivably small size, renders them very pene- 

 trating. They pass readily through matter opaque to 

 light, moving between the molecules, or even passing 

 directly through the latter, being smaller than the spaces 

 by which the atoms are separated within the molecule. 

 In their passage through substances they may collide 

 with and so dislodge other electrons, thus producing 

 ionization. The a particles, owing to their larger size 



2 Jean Becqnerel (Compt. Bend. Acad. Sci. Paris, 146: 1308. 22 Je 

 147: 121. 13 J], 1908) reports the experimental demonstration of the 

 ence of free positive electrons, but whether such electrons are invoh 



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