The architect of the cathedral in Cologne discusses weathering of the various stones that were used for a 700 year period. Only basaltic lava was found to be absolutely stable. The content of hydrochloric acid, sulfur dioxide and hydrofluoric acid in the air is primarily responsible for the destruction, as was determined by exact measurements. Deteriorated parts of the cathedral are replaced by new stones; weak areas are hardened with a silicic acid ester. R.W. -- AATA