HCR diary 15.4.1838 at Aders' "He, I should say they are annoyed at the difficulty in disposing of any of the pictures and now propose writing to Passavant at Frankfurt"
See Marquardt II p520 and n37. Johann David Passavant 1787-1861 art historian, saw Aders' collection in London in 1831 and described it in his book Kunstreise durch England und Belgien (1833). Not to be confused with the doctor Johann Karl Passavant (1790-1857) his brother?  whom Robinson met in Germany in 1829 (Marquardt II p 284 and n14)