ULTIMATE CONSTITUTION OF MATTER 83 



undulates down the list. And the importance of this com- 

 parison becomes apparent when it is noticed that the periods 

 of maximum and minimum prominence of all these dif- 

 ferent characters approximately coincide. 



To take an illustration from acoustics : Each tone of the 

 diatonic scale has its own rate of vibration, but each tone is 

 not a separate thing unrelated to all other tones. Some can 

 be sounded in harmony, others cannot. So also the gamut 

 of the elements may be divided into groups, which strangely 

 resemble octaves. Perhaps this analogy which has attracted 

 the attention of many chemical philosophers is more than 

 superficial; for the properties of the elements also depend 

 upon the vibration periods of their molecules. 



The observation that the properties of the elements, as 

 well as those of their compounds, are periodic functions of 

 the atomic weights of the elements that the properties of 

 the elements are determined by their atomic weights, that is 

 to say led Mendele*ef to classify them as follows: He 

 ruled a sheet of paper into eight vertical and twelve hori- 

 zontal columns. In the ninety-six places thus provided he 

 disposed the elements according to their atomic weights, the 

 lightest being in the top left-hand square, the heaviest in the 

 bottom right-hand square. The eight vertical columns he 

 termed "groups," the twelve horizontal columns "series." 

 The reader must not, however, think that this arrangement 

 could be carried out on any simple arithmetical basis. 

 There were, and still are, many difficulties and reasons for 

 uncertainty. For example 



(1) The eighth element in each alternate series exhibits 

 properties which would equally justify its inclusion as the 

 first of the next. It is therefore duplicated, and appears in 

 both. 



(2) Certain metals so closely resemble the duplicated 

 members that they have to be included with them in the 

 eighth column. Thus iron, nickel and cobalt appear in 

 the same square as copper. 



