It sounds a bit like a miniature mechanical pipe organ or a very breathy celeste. Legend once told that it could induce premature births and send you bonkers - which is probably why Donizetti originally used it for the mad scene in Lucia di Lammermoor.
The glass harmonica is an archaic oddity, but it has its modern-day champions, including French ondes-Martenot/glass-harmonic specialist Thomas Bloch, who joined the Hebrides for this enchanting programme.
Mozart wrote a handful of works for the glass harmonica and Bloch played a solo one here - the Adagio in C K356/617a - with surprising eloquence.
In Adagio and Rondo K617 and F-minor Fantasia K594 the strange voice became textural among strings and winds - an otherworldly cushioning that made the other instruments sound touchingly earnest in contrast.
The concert opened with Mozart's Oboe Quartet, beautifully led by Georgian soloist Giorgi Gvantseladze.
Also on the programme were two works by American composer George Crumb, who deals in sonic atmosphere via often gorgeous instrumental trickery. His Four Nocturnes were whispered but slightly too literal, while Vox Balaenae - 1970s music inspired by whale-song - was more absorbingly done with house lights down, masked musicians and some striking ensemble playing.