I've passed Cabinet Chaptal many times, as it is located on the corner just across the street from Jardin du Luxembourg. I'm not sure it's ever been open when I peered in, so I"ve just taken shots through the bars across the windows. I hope one day to go in and have a long look. Maybe I'll be lucky, and they will allow photos.
Cabinet Chaptal
1 rue de Fleurus
75006 Paris