What is the Law of octave?
An old observation that predated and foreshadowed the concept of Periodicity.
John Newlands was an English chemist of the 19th century. I just read in a bio that he actually fought for Garibaldi in the wars for Italian unification, so he must have been quite a remarkable character. As a chemist he noted an apparent Periodicity in the chemistry of the then known elements, when arranged by order of their atomic masses. Every eight element was known to have similar chemistry, and thus the law of octaves, after the 7 notes on the musical scale.