The name "harmonium" was patented in 1842, by Alexandre Debain. The instrument at this point had reached the "classic 4 rank" stsge, with a single keyboard, 4 sets of reeds, and the keyboard being split into bass and treble. I
"Debain's invention consisted of an instrument operated by air at a pressure above that of the surrounding atmosphere, and having four rows or ranks of free reeds. Each of these rows comprised reeds of different quality and included two 8ft, one 4ft, and one 16ft pitch. Although these rows went through the whole compass of the keyboard, each row was mechanically divided, not exactly in half, so that it was possible to select, for example, different stops for the melody than for the bass. From then onwards, this became the classical format of the 'four rank harmonium' as it was called by contemporary musicians and in contemporary tutors."