THE PERIODIC LAW 147 



proximately the same mass behave similarly because they vi- 

 brate similarly, while atoms of greater mass might vibrate 

 harmoniously with the smaller ones. It is difficult to explain, 

 according to this hypothesis of the "genesis of the elements," 

 why their number should be as limited as it is. But some 

 facts are known, vaguely pointing to the idea that the atoms 

 of elements within the same periodic group are capable of 

 vibrating at harmonically related rates, and that the great 

 majority of chemical and physical properties depend upon 

 atomic vibrations. It may, however, be argued that just as 

 violin-strings may be composed of different materials and 

 yet vibrate together according to common laws, so may the 

 elements be composed of as many individual materials and 

 still exhibit a periodic recurrence of properties, if the latter 

 depend upon the harmonic vibrations of the atoms. Until 

 much additional proof has been brought, the periodic law, 

 while furnishing a vague indication, cannot be taken as posi- 

 tive evidence of the qualitative unity of matter. 



In the table it will be found that the first group contains the 

 univalent elements, the second group those which are divalent, 

 and so on up to the seventh, where the maximum valency is 

 seven. The maximum valency of the elements of the eighth 

 group may be set at eight, but their compounds rarely ex- 

 hibit so high a valency, and in many other respects this eighth 

 group is rather anomalous and is taken as a transition group 

 between the seventh and the first. Thus the three elements, 

 copper, silver, and gold belong, with respect to many of their 

 properties, especially when uncombined, in the eighth group; 

 but their valency is usually low, and many of their salts are 

 so similar to those of sodium that it is often found expedient 

 to place them in the first group, in the positions occupied in 

 the table by their names inclosed in parentheses. The va- 

 lencies refer especially to the stable oxides. Stable compounds 



