6 2 6 SCIENCE PROGRESS 



accurate measurement, carried out by Millikan, of the charge 

 on an electron gives the value 4774 x io -10 electrostatic units. 

 This value depends, as a matter of fact, on the determination 

 of the Avogadro Constant N, to which we have already re- 

 ferred. 



As already stated, the charge on the electron is the smallest 

 quantity of electricity capable of independent existence. 

 Similarly the electron itself is the smallest portion of matter 

 capable of independent existence. The primary quality of 

 matter is mass, and an electron is the smallest mass of material 

 conceivable. No smaller mass has ever evidenced itself, 

 though it has been carefully sought for. It is known that the 

 mass of an electron is one eighteen-hundredth part of a single 

 atom of hydrogen. That is, the mass of an electron is 5 x io" 28 

 gram. 



This quantity is so very small that its removal from an 

 atom (of hydrogen, say) leaves us with a positively charged 

 residue, the hydrogen ion, which possesses a mass practically 

 identical with that of the neutral hydrogen atom itself. 



The remarkable conclusion at which we have arrived is, 

 then, this : the smallest particle of matter capable of existence 

 is known to us only in the electrified state — an unelectrified 

 electron is physically inconceivable — and the amount of elec- 

 tricity which it carries at the same time is the smallest quan- 

 tity capable of existence. Thi- "coincidence" of matter 

 and of electricity is exceedingly significant. In fact, Sir J. J. 

 Thomson has shown that the whole mass of the electron is 

 electro-magnetic. That is, in the electron, matter and elec- 

 tricity have become one. It is impossible to define the one 

 except in terms of the other. 



With this brief sketch of what we mean by electrical 

 charge and the concept of the electron, we may approach the 

 problem which we had set out to consider, namely, the struc- 

 ture of the atom. 



Atoms, as usually met with, are electrically neutral. They 

 cannot consist, therefore, of electrons alone, for these would 

 give rise to negatively charged systems, and further an assem- 

 blage of electrons, owing to their mutual repulsions, could 

 not possibly produce a stable arrangement. An atom must 

 contain at the same time some source of positive electricity. 

 At present we know relatively little about the positive elec- 



