hiatuses

— That many pretended cognoscenti in music are totally ignorant of such natural principles, I am convinced; for I have heard a lady play gracefully a lesson of Clementi's, who knew nothing about it, but how a man, of an investigating mind, can see the black keys of a harpsichord, and observe the two hiatuses in the rows of every octave, and not ask himself why, is to me unaccountable.