CHAPTER XIII 

 R A D I () - A C T I V I T Y 



THE ATOM IN DISSOLUTION 



" From liarniony, from lieavenly harmony 

 This universal frame began ; 

 \Vhen nature underneath a heap 

 Of jarring atoms lay." 



Dryden. 



The various manifestations of energy already dealt with have 

 all been associated with matter in the form of small aggregates 

 (colloids), atoms, or ions (charged hydrated radicles). Chemists 

 once defined the atom as the smallest non-divisible portion of 

 matter. Needless to say, many scientists were content to be 

 decryed as old-fashioned and refused to accept this opinion of 

 the atom. My old teacher. Prof. John Ferguson, would allow 

 no one to refer to atoms. He preferred the more cimibrous but 

 exact term " Combining Proportions." Modern work has con- 

 firmed these opinions of the atom. Physicists are now interested 

 in the structure of the atom. No longer is it considered as non- 

 divisible. No longer does it remain as fundamental. Of what 

 then does the atom consist ? Many and varied are the present- 

 day theories of its structure, but in general most schemes are 

 similar. It is supposed to consist of a number of smaller units — ■ 

 electrons, all moving rapidly, eccentrically and regularlij round a 

 central positive charge or nucleus. An electron is nothing more 

 than a unit charge of negative electricity. The number of electrons 

 in each ring is definite and may undergo alteration in definite 

 quanta only. 



(1) Not more than a certain number of electrons can continue 

 in stable motion in one ring. If more are added the system breaks 

 up into two or more rings. 



(2) If the orbital velocity of the electronic rings exceeds or falls 

 below a certain critical value, the electrons are rearranged to 

 ensure stability for that speed. 



A model may make this clearer. The outer particles may be 

 represented by a number of exactly similar sewing-needles, 

 magnetised simultaneously in a solenoid. They are floated verti- 

 cally in a small trough (Fig. 39), by having, say, their A'^ poles 



155 



