“. . . I arrived in a city that always makes me think of a whited sepulcher.” (44)

A sepulcher is a place of burial, more commonly called a tomb. The phrase “whited sepulcher” has come to connote a person who, in outward appearances, is virtuous and good while is inwardly exactly the opposite – a hypocrite, fitting with one of the major themes of Conrad's tale. For Marlow to see Brussels, as well as the company Company, as whited sepulchers is significant: while the city puts off an image of goodness it instead houses a monster - the Company - which claims to want to colonize the Congo and make it a better place, all they in fact care about is procuring enough ivory to turn a tidy profit.

Source:

"Whited Sepulcher - Fefinition of Whited Sepulcher by the Free Online Dictionary, Thesaurus and Encyclopedia." Dictionary, Encyclopedia and Thesaurus - The Free Dictionary. Web. 24 Aug. 2009. http://www.thefreedictionary.com/whited+sepulcher.