212 PHYSICAL SCIENCE 



deal with the intimate structure of the atom, as 

 the more fundamental unit. 



The essence of Dalton's great conception was 

 that the relative chemical combining weights of the 

 different elements lead directly to a knowledge of 

 the relative weights of the atoms of those elements. 

 Since Dalton's time it has been recognised that 

 the atoms, in the chemical sense of the word, of 

 different elements must have different weights 

 and different properties. If, then, we look for 

 some common constituent composing the different 

 elementary substances known to chemistry, we 

 must look within the atom ; we must cease to 

 regard it as the ultimate unit, and examine the 

 internal structure of the atom itself; we must 

 abandon the etymological meaning of the word, 

 retaining it only for its historic associations. 



On arranging the elements in order of their 

 atomic weights, Mendeleeff discovered that periodic 

 relations become apparent between the physical 

 and chemical properties, elements with similar 

 properties recurring at constant intervals. This 

 periodicity was so marked a feature that it was 

 possible to arrange the elements in groups, in 

 which the various properties were possessed by 

 the individual members to a greater or less extent, 

 according to their position in the groups. It was 

 even possible successfully to predict the atomic 

 weight, properties, and compounds of undiscovered 

 elements from knowledge of the behaviour of their 

 neighbours, which were situated round empty 

 spaces in the periodic table. 



The periodic law suggests a common origin 



for the elements, and indicates that, as we pass 



rom light to heavy atoms, we are going from 



