Joris Verdin Is Léon Boëllmann’s Heures Mystiques organ music?
Het ORGEL 97 (2001), nr. 5, 12-19 [summary]

Léon Boëllmann (1862-1897) liked the harmonium. Many pieces from Heures Mystiques are meant to be played on harmonium, as are large parts of César Frank’s L’Organiste. This can be concluded from the Preface to Heures Mystiques, from the way the pieces are composed, from the registration indications and several dynamic aspects.

Some of the pieces can be played on the organ, although this may result in a loss of dynamic qualities. Boëllmann demonstrated with the Suite Gothique and the Deuxième Suite that he could write proper organ music as well.