MATTER, SPACE, AND TIME 217 



Thus the periodicity in the properties of the 

 elements is reproduced in the number of electrons 

 in the outer layer. It is clear that these electrons 

 are specially concerned in chemical reaction. The 

 valencies of the elements, that is, the number of 

 simple atoms like hydrogen or chlorine with which 

 these atoms will combine, rise from one to four 

 as we pass from lithium to carbon or from sodium 

 to silicon. Nitrogen and phosphorus can be both 

 pentavalent and trivalent, oxygen and sulphur 

 are divalent, fluorine and chlorine monovalent, 

 while neon and argon do not react. Hence it 

 seems that the valencies depend on the number 

 of these outer electrons from i to 4, and decrease 

 again as the numbers rise higher. 



We may picture chemical combination between 

 two atoms as due to the forces between their 

 nuclei and the free electrons 

 of their outer layers. The 

 simplest case, the union of 

 two hydrogen atoms to form 

 a hydrogen molecule, may be 

 represented by Fig. 35, in 

 which A and B are the positive nuclei, and a and 

 /3 two negative electrons. A repells B and a 

 repells A while A and B attract both a and ^. 

 The electrons may then be regarded as common 

 to both atoms, and, on this theory, that arrange- 

 ment is the meaning of chemical combination. 



At first sight we may well say that Thomson's 

 corpuscle — one of the latest conceptions of science 

 — does but carry us back to the ideas and specu- 

 lations of Democritus, and justify the glorification 

 of those ideas in the poem of Lucretius, though 



