LECTURING on October 12 to the Association of Architects, Surveyors, and Technical Assistants, Dr. W. H. J. Vernon, of the Chemical Research Laboratory of the Department of Scientific and Industrial Research, referred particularly to the composition, properties, and artificial reproduction of the attractive green patina which appears on copper on long exposure to the atmosphere. In this respect, copper is unique among roofing materials. Urban samples of patina have been found to consist essentially of basic copper sulphate, and not of copper carbonate as was previously supposed. Evidently sulphur dioxide is the predominant atmospheric factor in its formation, a view which is supported by the fact that the patina is more difficult to produce in rural districts than in towns. The material first formed is the normal copper sulphate which, being soluble in water, causes disfigurement of stonework underlying copper or bronze statues. After about seventy years, however, the film has been converted into a substance of composition similar to that of the mineral brochantite, the only basic copper sulphate which occurs in Nature. Thereafter the composition appears to remain constant for periods of exposure up to three hundred years. The attainment of a uniform green colour by atmospheric exposure is preceded by a period of ten to twenty years, during which the metal presents an unsightly black appearance owing to the presence of sulphide, oxide, and carbonaceous matter. The period of unsightliness can now be eliminated by subjecting the copper to electrolytic corrosion under controlled conditions; after 15 minutes an insoluble patina of good green colour is produced, and this, after exposure to the air, is found to have approximately the constitution of brochantite. By far the best results are obtained with arsenical copper, and it is significant that all the old copper roofs which have been analysed are also arsenical.