William Pole

“ The number of musical sounds producible, all differing in pitch, is theoretically infinite, and is practically very large; so that in a single octave a sensitive ear may distinguish 50 to 100 different notes. But if we were to take a number of these at random, or if we were to slide by a continuous transition from one sound to another considerably distant from it, we should not make what we call music. In order to do this we must use only a certain small number of sounds, forming a determinate series, and differing from each other by well-defined steps or degrees. ”

“ The introduction of chromatic notes gave rise to a third kind of semitone, as from C to C♯ or from G to G♭; this is called a chromatic semitone and has a less magnitude than the diatonic one.
Finally came the great simplification of music by dividing the octave into twelve equal intervals, each of which was called a mean semitone; thus abolishing practically the difference between the diatonic and the chromatic values. A semitone may now be considered, in practical music, as simply the interval between the sounds given by any two adjoining keys on a well-tuned piano. ”

“ For modern European music, in ascending from any note to its octave above, we employ, normally, a series of seven steps of unequal height, called the diatonic scale, with the power of interposing, accidentally, certain intermediate chromatic steps in addition. The diatonic scale is of Greek origin, having been introduced about the middle of the sixth century b.c. ”